Mark is right,

but it is kind of interesting because a is an abstract value.

what my algebra computations conclude is

y=(-6a+3)x+3a^{2}-3 is tangent line formula

let's say f(x)=-3x^{2}+3x-3 and a=11

then y=(-63)x+360 would be tangent line to curve at (11,-333)