Hello there!
The way to figure out what sides lengths are admissible to be the proper side lengths of a right triangle, they must follow the 3/4/5 rule.
The 3/4/5 rule is known as the Pythagorean Theorum, or also known as
"a^2 + b^2 = c^2"
First, let's try 18 for A, 24 for B, and 30 for C.
18^2 + 24^2 = 30^2
Simplify.
324 + 576 = 900
900 = 900.
This does apply to the Pythagorean Theorum, so the lengths can be equal to a right triangle.
I hope this helps!