When finding these problems we always have to do the opposite of the operation shown.
In this case its:
5x + 15 = 45
Subtract 15 from both sides (we start off with constants and numbers that are being shown) The reason why its subtraction is because since your adding 15 to 5x the opposite of that is subtraction so use the number shown by itself and subtract it from both sides.
5x = 30
Divide by 5 on both sides since the opposite of 5x is division (only use the number that is shown) We have to get rid of all the numbers on the left.
