if f(x) = ax + b and f(2) = f(4) then a = ?

Question

i cant seem to figure out if i plug in 2 or 4 into the eq.

i no the answer is 0 but how do i strt?

f(2) = f(4) --->

2 a + b - 4 a + b --->

2 a = 4 a --->

(4 - 2) a = 0 --->

2 a = 0 ---->

a = 0