My solution works and i think its a bit easier hope it helps !

alert("Hello and welcome,let's do little math !");

//instantly making strings into numbers

const firstnumber = parseFloat(prompt('Enter your first number please'));
const secondnumber = parseFloat(prompt('Enter your first number please'));

//declaring the calculations

const sumnumbers = firstnumber + secondnumber;
const minusnumbers = firstnumber - secondnumber;
const dividenumbers = firstnumber / secondnumber ;
const multiplenumbers = firstnumber * secondnumber;

//putting tags on the page for printing

const main = document.querySelector('main');
main.innerHTML = `
<h1>your numbers are ${firstnumber} and ${secondnumber}, so these are the calculations:</h1>
<br>
<p> the sum of ${firstnumber} + ${secondnumber} = ${sumnumbers}
<p> the minus of ${firstnumber} - ${secondnumber} = ${minusnumbers}
<p> the divide of ${firstnumber} / ${secondnumber} = ${dividenumbers}
<p> the multiple of ${firstnumber} * ${secondnumber} = ${multiplenumbers}
`;