In the Pythagorean Theorem, a^2+b^2=c^2. The measurements provided here are 30, and 18. 30 is c and 18 is either a or b (it doesn’t matter) 30 is c because it is specified as a diagonal. In reverse to the Theorem, it would be 30^2-18^2= a^2 that would be 900-324. You would then square root what you got for “a” to find your answer. It would be 24.