given a = 15 m and b = 9 m and c = 1
so you can find the hypo. of the right triangle on the right
=15^2 + 9^2
=225 + 81
=306
so hypo. of the right triangle on the right = √306
the hypo of the right triangle is long leg of the right triangle on the left
so
x^2 = 1^2 + 306 = 307
x = √307
x = 17.5 m