Identify f. Help please! I am so confused.

Triangle XYZ is a right triangle.
What value should f be to yield 90 degrees?
6(3f - 15)°.
Let f be 8, 10, 12, 15.
When f = 10, we get 90 degrees for angle Z.
Z = 6(3•10 - 15)
Z = 6(30 - 15)
Z = 6(15)
Z = 90
Answer: Choice B.