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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The worksheet format and Answer Key enable self-correction with minimal dependence or direct help from a chess teacher, coach, or parent.
Answer – Diagram 46:
1. Ra8+ Qxb8
2. Ra/dxd8+ Be8
The slash (/) is used to show that either the Rook on the a-file or d-file (Ra/dxd8+) takes on d8.