F = 16 + 15 ... it says "If you are 16 years old now", so x = 16
F = 31
You'll be 31 years old 15 years from now.
The equation used is either F = x + 15 or F = 16 + 15 depending on how you do it. You can also manipulate the equation to get either F - x = 15 or F - 16 = 15. So there are many ways to do this.