Answer:
Well, it's Pythagoras Theorem
hyp²= opp² + adj²
hyp (hypotenuse)
opp (opposite)
adj (adjacent)
Since, we already have hypotenuse and opposite
15²= 9² + adj²
making adjacent the subject of formula
15²-9² = adj²
adj² = 225-81
adj² = 144
make adjacent subject formula by removing the ²
so,
adj = 12