There are several information's already given in the question. Based on those given information's the answer can be easily deduced.
Length of the hypotenuse of the triangle = 18 inches
let us assume the length of the other 2 sides = x
The
x√2 = 18
x = 18/√2
= 18/1.414
= 12.73 inches.
I hope that this is the answer that you were looking for and the answer has come to your desired help.