Hey there!
Solve using pythagoras theorem (it's given that is triangle is right angled. Pythagoras theorem only works in the case of right triangles) ⇨ Hypotenuse² = Base² + Altitude ²
Here,
Base = 9 mm
Hypotenuse = 15 mm
Altitude = b
Hypotenuse² = Base² + Altitude ²
15² = 9² + b²
225 = 81 + b²
225 - 81 = b²
144 = b²
√144 = b
12 mm = b
Hope it helps ya!