a suitcase measures 24 inches long and the diagonal is 30 inches long how much material is needed to cover one side of the suitcase

I believe you're supposed to solve for the other missing side. If you know a side (a) and the diagonal or also known as the hypotenuse (c), you will be able to solve for the other side using the Pythagorean theorem.
a^2+b^2=c^2 Substitute the values for a and c
24^2+b^2=30^2 Solve for the powers
576+b^2=900 Subtract 576 from both sides
b^2=324 Solve for the root of 324
18=b
You would need 18 inches of material to cover one side of the suitcase.