Find the missing side length using the Pythagorean Theorem.
a²+6² = ²

The missing length is 18
Using Pythagorean rule
Hyp^2 = Opp^2 + adj^2
Opp^2 = hyp^2 - adj^2
Opp^2 = 30^2 - 24^2
Opp^2 = 900 - 576
Opp^2 = 324
Opp = sqrt ( 324 )
Opp = 18