The whole thing less the little piece.

Notice the two possible derivatives of the volume of a right circular cyllinder.

1)

2)

3)

See that 'r' in 2 and 3? That is the same r. You had '2-x' in the first one, you MUST have '2-x' in the second.

By the way, you get awesome and courage points for trying it both ways. Good work. :-)