1...bxa6 leads to 2.Qb6+ Ka8 3.Qxa6+ Ra7 4.Qxc8#
Black Resigns (no defense against Ra8 leading to Qa5#)
There is a defense against 2.Qa4 and prevents Ra8+ and Qa5# But it is still a loss for black.
Analysis by Gordon B. Elo rating 2681 Starting from original position!!! 1.Rxa6+! Kb8 2.Qa4 Rxc5!! 3.dxc5 Qxc5+ 4.Kh1 Kc7 5.Ra7 Kd6 6.Ra5 Qxa5 7.Qxa5 Ke7 8.Qc5+ Ke8 9.Rxb7 Rd1+ 10.Ng1 Nd7 11.Qb4 Rd3 12.Qa4 Rd5 13.Nf3 f6 14.Nd4 Ke7 15.Kg1 g5 16.Qa7 Ke8 17.Nxe6 Rd1+ 18.Kf2 Rd2+ 19.Ke1 Rd3 20.g4 g6 21.Ke2 Rd6 22.Nc5 Rd5 23.Nxd7 Rd2+ 24.Kxd2 Ke7 25.Qc5+ Kf7 26.Nxf6+ Kxf6 27.Qe7#