My solution with comments

/*
  Simple script to display some
  math on the html page with numbers
  from user input.
*/

alert("Let's do some math!");

// store prompt input as number type using +
const num1 = +prompt("Enter first number");
const num2 = +prompt("Enter second number");

// checks that variables are valid numbers, and num2 is not 0
if ( isNaN(num1) || isNaN(num2) || num2 === 0 ) {

  // if not valid, alert user and do not continue block
  alert("Invalid input, please reload and try again.");

// if valid continue to else statement
} else {

  // create message using multiline template literals
  const message = `
    <h1>Math with the numbers ${num1} and ${num2}</h1>
    ${num1} + ${num2} = ${num1 + num2}
    <br>
    ${num1} * ${num2} = ${num1 * num2}
    <br>
    ${num1} / ${num2} = ${num1 / num2}
    <br>
    ${num1} - ${num2} = ${num1 - num2}
  `;

  // write message to document
  document.write(message);
};