Answer: If it is a right triangle, as must be the case in this question, the longest side will always be opposite the right angle.
Step-by-step explanation: If you have diagrams or examples to look at, it is evident from looking. The short sides are "legs" connected to the right angle. The longest side is opposite, not connected.
The Pythagorean Theorem also helps to prove it. The square on the hypotenuse is equal to the sum of the squares on the opposite sides. If it is the SUM, it must be larger than either of the other numbers.