Help with this separable differential equation problem

In the elementary chemical reaction, single molecules of two reactants A and B form a molecule of the product C: A + B -> C. The law of mass action states that the rate of reaction is proportional to the product of the concentrations of A and B:

d[C]/dt = k[A][B]

Thus, if the initial concentrations are [A] = a moles/L and [B] = b moles/L and we write x = [C]. then we have:

dx/dt = k(a - x)(b - x)

a) Assuming that a != b, find x as a function of t. Use the fact that the initial concentration of C is 0.

b) Find x(t) assuming that a = b. How does this expression for x(t) simplify if it is known that [C] = 1/2a after 20 seconds?

I honestly have no idea where to start.