9 feet.
To find the answer, we must make this a right triangle. To do this, we will remove the 8 feet on the left and subtract that from the 20 feet parallel from it. This will leave us with a height of 12 feet and the slope being 15 feet long. To solve for the unknown, we will use the Pythagorean Theorem, which states that a^2 + b^2 = c^2. Since we already know what side c is (the slope, 15 feet) and are trying to solve for either a or b (it can be either one because they are the same), we will subtract. 15^2-12^2 = 81. Now we will find the square root of 81 because it is the number squared, and we just want the number by itself. Since the square root of 81 is 9, we know that the length of ? is 9 feet.
