Try writing some code:
  1. start a new html document
  2. Inside the body, put your name inside an H1 tag
  3. Add a script tag: 
    <script>

</script>
  4. Declare a variable named answer
  5. Use the prompt() function to get a value for answer 
    answer=prompt("Enter a number", "0");
  6. alert the answer:
    alert (answer);
  7. Get a second number
  8. Write the second number to the screen using document.write 
    value2=prompt("enter another number", "0");
document.write (value2);
  9. alert the sum of the 2 numbers
    alert("the answer is" + answer+value2)
  10. What's wrong??
    You can't mix strings and numbers
  11. Try the following code to see what is happening 
      alert (parseInt(answer)+parseInt(value2));
  alert ("answer:" + parseInt(answer)+parseInt(value2));
  12. Now, lets fix the code to work the way we want 
      sum= parseInt(answer)+parseInt(value2);
  alert ("answer:" + sum);
Link to solution