f(x) = k(a - x)(b - x)
Find the derivative:
a) f'(x) = k(a - x) - k(b - x)
b) f'(x) = k(a - x) + k(b - x)
c) f'(x) = -k(a - x) + k(b - x)
d) f'(x) = -k(a - x) - k(b - x)