Help!!

I don't understandwhat I'm doing wrong. What does the (a) stand for in the function

$f(a) = 3a – a^2$

a is a variable and can be equal to any number or any mathematical expression.

It is the same as 

$f(x) = 3x – x^2\\ f(x) = – x^2+3x\\ f(x) =-x(x-3)$

This would graph as a concave down parabola with roots of 0 and 3