Black promotes the a-pawn to a Queen in 2 moves!
See Answer below.
Diagrams from John Bain’s best-selling book,
Chapter 10 Promoting Pawns –
PROMOTING A PAWN, often called Queening a Pawn, is one of the most powerful moves in chess.
Promoting a pawn often results in being a Queen ahead! Because of this, sacrificing one or more pieces to make it possible to promote a pawn may be the best strategy in many positions.
Promoting a pawn often takes two or more moves to set up, as shown in Diagram 280.
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.
This book is ideal for independent seat work as well as for homework and classroom instruction.
The worksheet format and Answer Key enable self-correction with minimal dependence or direct help from a chess teacher, coach, or parent.
Answer – Diagram 295:
1. … Qxd1
2. Qxd1 a1=Q