True or False and why. If f(x)= 2x-9 then f(3)=-3.

Question

The answer was true.  I have no idea why?  There are 2 unknowns 

Answer 1

f(3) simply means we need to replace the x with 3. So since f(x)=2x-9 then f(3)=2(3)-9 which is equal to -3.