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If I am picturing this correctly, a right triangle is formed, with the diagonal of 75' being the hypotenuse.
x = one leg (vertical height)
x + 15 = other leg (horizontal distance)
Use the Pythagorean theorem,
c^2 = a^2 + b^2
75^2 = x^2 + (x + 15)^2
Solve for x