Answer:
√65
Step-by-step explanation:
Use Pythagorean Theorem (works for only right triangle)
a² + b² = c²
a is short leg
b is longer leg
c is the hypotenuse (longest leg)
4² + b² = 9²
(After, solve like another algebraic equation) 16 + b² = 81
b² = 65
b = √65