The length is: 24 meters
Here's why
So the hypotenuse theory or aka A^2 + B^2 = C^2 lets us know this:
A = 18
B = What we are looking for
C = 30
So before we apply these we're going to do some magic:
B = sqrt(-A^2 + C^2)
Now just put in the numbers
B = sqrt(-18^2 + 30^2)
B = sqrt(576)
B = 24 meters