You would use the Pythagorean Theorem
a^2 + b^2 = c^2
The sides 'a' and 'b' are the legs of the right triangle. The side c is the hypotenuse or the longest side. In this case, the longest side is c = 61. The other two sides are a = 11 and b = 60. The order of 'a' and 'b' does not matter.
Let's use the substitution property to plug in the given values mentioned and we will get...
a^2 + b^2 = c^2
11^2 + 60^2 = 61^2
121 + 3600 = 3721
3721 = 3721
We get the same number on both sides; therefore, the original equation is true when (a,b,c) = (11,60,61).
This all confirms we do have a right triangle. The right angle is opposite the hypotenuse. This is due to the rule that the largest angle of a triangle is always opposite the largest side.