In the original equation, x is a variable, hence f(x). If that term in the parenthesis changes, such as to 0 or -2, we will plug in that number in place of the current variable.
f(0) = 4(0)^2 + 3 = 0 + 3 = 3
f(-2) = 4(-2)^2 + 3 = 4(4) + 3 = 16 + 3 = 19
Hope this helps! :)
