f(x) = x^2 - 3x
f(f(x) ) = (x^2 - 3x)^2 - 3(x^2 - 3x)
So
(x^2 - 3x)^2 - 3(x^2 - 3x) = x^2 - 3x let x^2 - 3x = m and we have that
m^2 - 3m = m
m^2 - 4m = 0
m(m - 4) = 0
So m = 0 so x^2 - 3x = 0 so x(x - 3) = 0 so x = 0 or x = 3
And
m - 4 = 0 so x^2 - 3x - 4 = 0 so (x - 4)(x + 1) = 0
Setting both of the last two factors to 0 and solving for x gives that x = 4 and x = -1
So....x = { -1, 0, 3, 4 }