Learn what a functional value, image and pre-image are, then practice solving.
If f(x) is a function and for a given value of x, the corresponding value y is obtained, then the value y thus obtained is called the functional value.
For example, if f(x) = x + 5 and x = 2, then the value of the function is f(2) = 2 + 5 = 7. f(x) is called the image of x, and x is called the pre-image of y or f(x).