Since the triangle is a right triangle, we can use the pythagorean theorem to solve for the one remaining side.
Plug in your values
a = 9
b = ?
c = 15
9^2 + b^ = 15^2. Square root 9 and 15
81 + b^2 = 225. Subtract 81 from each side.
b^2 = 144. Take the square root of each side.
b = 12.
Now we know the length and width of the rectangle.
Perimeter = 2(12 + 9)
Perimeter = 42 in
