So when a variable (which is x, y or whatever letter) is sitting beside a number that means you'll be multiplying.
So 3 multiplied by 5 (since we know x is equal to 5) is 15.
And since we know 3x (which can also be written as 3(5) if that helps) is equal to 15 our math problem would look like this
15 + 8.
So 15 plus 8 is equal to 23.
I hope this helps :)
