It looks like you want to find the short leg (x) of a right triangle with long leg 15 and hypotenuse 18. The Pythagorean theorem gives the relationship between the sides of a right triangle.
18² = 15² + x²
324 = 225 + x²
x² = 324 - 225 = 99
x = √99 ≈ 9.9
The appropriate choice is ...
b) 9.9
