How come I can combine interpolation and concatenation

// 1. Attach this file -- math.js -- to the index.html file using a <script> tag

// 2. Add an alert to announce the program with a message like "Let's do some math!"
alert("Let's do some math!");

// 3. Create a variable and use the prompt() method to collect a number from a visitor
const getFirstNum = prompt("Enter a low number: ");

// 4. Convert that value from a string to a floating point number
const convertFirstNum = parseFloat(getFirstNum);

// 5. Repeat steps 3 and 4 to create a second variable and collect a second number
const getSecondNum = prompt("Enter a higher number: ");
const convertSecondNum = parseFloat(getSecondNum);

// 6. Create a new variable -- message -- which you'll use to build
//    a complete message to print to the document
//    Start by creating a string that includes <h1> tags as well
//    and the two input numbers. The string should look something like this:
//    "<h1>Math  with the numbers 3 and 4</h1>" where the two numbers are
//    the values input from the user. Use string concatenation to create this
//    and make sure you actually perform the math on the values by 
//    using the + symbol to add their values together
const addResult = convertFirstNum + convertSecondNum; 
const subResult = convertFirstNum - convertSecondNum; 
const divResult = convertFirstNum / convertSecondNum; 
const mulResult = convertFirstNum * convertSecondNum; 
const message = "<h1>Math with the numbers "+convertFirstNum+ " and " +convertSecondNum+"</h1>"

// 7. Add another string to the message variable. 
//    The string should look something like this after concatenation:
//    "3 + 4 = 7"
   +  `<br> ${convertFirstNum} + ${convertSecondNum} = ${addResult}`
// 8. Add a linebreak tag -- <br> -- to the message variable
  +`<br> ${convertFirstNum} - ${convertSecondNum} = ${subResult}`

// 9. Continue to add to the message variable to include strings
//    demonstrating multiplication, division and subtraction
//    For example:
//    "3 * 4 = 12"
//    "3 / 4 = 0.75"
//    "3 - 4 = -1"

 +`<br> ${convertFirstNum} * ${convertSecondNum} = ${mulResult}`
 +`<br> ${convertFirstNum} / ${convertSecondNum} = ${divResult}`

// 10. Use the document.write() method to print the message variable 
//     to the web page. Open the finished.png file in this workspace
//     to see what the completed output should look like

 document.write(message);