Assuming there is not loss of energy due to frinction (which is an idealization), you can use the conservation of energy law in the form:
Mechanical energy at top = mechanical energy at bottom.
Mechanical energy at top = potential energy = m * g * h
Mechanical energy at bottom = kinetic energy = m * v^2 / 2
=> m *v^2 / 2 = m * g * h
=> v^2 / 2 = g * h
=> v ^2 = 2gh
=> v = √(2gh) = √(2 * 9.8 m/s^2 *30m) = 24.25 m/s
Answer: 24.25 m/s