Answer:
Full question is attached. 
Tommy's reasoning is wrong because the hypotenuse is not 5 feet. 
A skateboard ramp resembles a right angles triangle so its sides resemble that of a right angled triangle. 
The height of 3 feet would be the height of the triangle. 
The length of 5 feet would be the length of the triangle. 
The piece of plywood that would cover the top of the frame would be the hypotenuse. 
In using the Pythagoras rule, the c is the hypotenuse but Tommy made the mistake of assuming that the hypotenuse was b which is where the error came from. 
Instead of solving for b, they should have been solving for c which is the hypotenuse. 
a² + b² = c²
3² + 5² = c²
9 + 25 = c²
c² = 34
c = √34
c = 5.83 feet
They should use a plywood of 5.83 feet not 4 feet.