If 3 is one zero of the polynomial f(x) = 9x? - 3(a - 1)x + 5, then the value of a is?

assuming 
f(x) = 9x^2  -3(a-1)x + 5
if 3 is a zero, f(3) = 0
thus
f(3) = 9(3)^2 - 3(a-1)*3 + 5 = 0
81  - 9(a-1) + 5 = 0
81 -9a +9 +5 = 0
9a = 95
a = 95/9

if it is x^n and x^2 do the same replacement.