Question

For each function, what is the output of the given input? . . For f(x) = 4x + 7, find f(4)

Answer 1

Look at the LHS in f(x) = 4x + 7, ie, f(x). Compare it with f(4). You see that we've put 4 in place of x. 
To find f(4), put 4 in place of x in RHS. Simple. 

f(4) = 4 * 4 + 7 = 16 + 7 = 23.