15. The left triangle is similar to the whole triangle. The x of the small triangle is proportional to the hypotenuse of the large triangle (9+16 =25). the x of the large triangle is proportional to the 9 unit leg of the small triangle. this creates the proportion [tex] \frac{x}{9} = \frac{25}{x} [/tex]. Cross multiply to get [tex] x^{2} =225[/tex] take the square roots of both sides to get x=225
