the solution on this lesson is 1.Rxd8 to removes the defender of f6 and then 1...Qxd8 to keeps f6 guarded, after that white bring the other rook to attacks the black queen, planning to chase it away from guarding f6, 2. Rd1 Qe7 3. Rd7! , and white is winning.
http://www.chess.com/chessmentor/view_lesson?id=6898
the solution on this lesson is 1.Rxd8 to removes the defender of f6 and then 1...Qxd8 to keeps f6 guarded, after that white bring the other rook to attacks the black queen, planning to chase it away from guarding f6, 2. Rd1 Qe7 3. Rd7! , and white is winning.
My question is, why not 1... f6 ?
it's not explained on the box note