Please help with JS calculator...I'm stuck!

Code Feedback
1

I’m trying to figure out how to get my calculator to perform an operation after hitting equal button. For example, 1+2=4 after pressing ‘equal’ to get 4, I want to press ‘+’ and add the result to a number. At this time it would display the ‘+’ sign instead of doing the operation. Also, I would like to allow user to press the key ‘+’ and it displays only once. At the moment, keys like ‘.’, ‘+’ etc…can be displayed multiple times. I’m new at coding. Any help is appreciated. Here’s the link to my code pen. https://codepen.io/QuidProQuo/full/VMbPgj

JS codes below

var show = document.getElementById('display');

var reset = false;
function clear(){
	if(reset){
		show.value = '';
		reset = false;
	}
}

function toScreen(x){
	clear();
	show.value += x;
	if (x === 'c'){
		show.value = '';
	}
}

function answer(){
	x = show.value;
	x = eval(x);
	show.value = x;
	reset=true;
}

function power(){
	x = show.value;
	x = eval(x*x);
	show.value = x;
}

function backspace(){
	var num = show.value;
	var len = num.length-1;
	var newNum = num.substring(0,len);
	show.value = newNum;	
}

function percent(){
	x = show.value;
	x = eval(x/100);
	show.value = x;
}

function opposite(){
	var n = show.value;
	n = n * -1;
	show.value = n;
}

 function sqrt(){
	var number = show.value;
	var ans = Math.sqrt(number);
	if (number < 0) ans="Try Again...";
	show.value = ans;
}

function pie(){
	var pi = show.value;
	var result = Math.PI;
	show.value = result;
}

html codes

	<h1>Javascript Calculator</h1>
	<div id = 'calculator'>
		<!--create a form-->
		<form>
			<input type='text' id='display' disabled>
			<br>

			<input type='button' value='C' id='keys' onclick='toScreen("c")'>
			<input type='button' value='CE' id='keys' onclick='backspace()'>
			<input type='button' value='x^2' id='keys' onclick='power()'>
			<input type='button' value='+' id='keys' onclick='toScreen("+")'>
			<br>

			<input type='button' value='9' id='keys' onclick='toScreen("9")'>
			<input type='button' value='8' id='keys' onclick='toScreen("8")'>
			<input type='button' value='7' id='keys' onclick='toScreen("7")'>
			<input type='button' value='-' id='keys' onclick='toScreen("-")'>
			<br>

			<input type='button' value='6' id='keys' onclick='toScreen("6")'>
			<input type='button' value='5' id='keys' onclick='toScreen("5")'>
			<input type='button' value='4' id='keys' onclick='toScreen("4")'>
			<input type='button' value='*' id='keys' onclick='toScreen("*")'>
			<br>

			<input type='button' value='3' id='keys' onclick='toScreen("3")'>
			<input type='button' value='2' id='keys' onclick='toScreen("2")'>
			<input type='button' value='1' id='keys' onclick='toScreen("1")'>
			<input type='button' value='/' id='keys' onclick='toScreen("/")'>
			<br>

			<input type='button' value='0' id='keys' onclick='toScreen("0")'>
			<input type='button' value='.' id='keys' onclick='toScreen(".")'>
			<input type='button' value='=' id='equal' onclick='answer()'>
			<br>