Just evaluate V(r) - V(r+1)
V(r) as the expression itself,, then replace r with r+1 to get V(r+1)
So it will be A/(r+2) - B/(r+1)
Then simplify usingthe common denominator r(r+1)(r+2)
You will end up getting Ar- Br - 2B
Obvoiusly B=1 and A = 4 to get 3r-2