Integrating with absolute value?

Let f(a) = int 0 to 2 |x(x-a)|dx for 0<=a<=2

find the function f(a)

The solution says to separate it into two integrals from 0 to a then to a to 2. Then take the negative of both then add? I don't get it. Why do you have to take the negative of each? Please explain!