#### Answer

The unknown length in the window frame is $= 5$ ft.

#### Work Step by Step

Let $H =$ hypotenuse of the triangle / unknown length in the window frame
$a^{2} + b^{2} = c^{2}$
$4^{2} + 3^{2} = H^{2}$
$16 + 9 = H^{2}$
$25 = H^{2}$
$H = \sqrt 25$
$H = 5$ ft