White checkmates in 3 moves.
See Answer below.
Chapter 2 BACK RANK COMBINATIONS
A BACK RANK COMBINATION is based on the enemy King being trapped on the back rank by its own pawns and the fact that the back rank in not protected by a Rook or Queen. Some back rank combinations are simple, as in introductory Diagram 32. Others take two or more moves to set up.
Diagrams from John Bain’s best-selling book
Chess Tactics For Students is a workbook for intermediate and advanced students containing 434 carefully selected problems in worksheet format. Chess Tactics has been extensively field tested with elementary, middle-school, and high-school students over a four-year period.
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Answer – Diagram 54:
1. Qxa8+ Qc8
2. Qxc8+ Ne8
The slash (/) is used to show that either the Queen or the Rook on e2 (Q/R) captures on e8 for checkmate.