First, distribute f(x). k(x - k) = kx - k^2. Now evaluate f(4) and f(3):
f(4) = k(4) - k^2 = 4k - k^2
f(3) = 3k - k^2
Now subtract:
(4k - k^2) - (3k - k^2)
= 4k - k^2 - 3k + k^2
= 4k - 3k = k, choice (B).