Gravitation potential energy = mass of ball x acceleration due to gravity (9.81 m/s^2) x change in height
Assuming the ball started from the ground, it's gain in gravitational potential energy will be the following when you sub in the given values:
Ep = mgh
Ep = 4.5 x 9.81 x 0.135
Ep = 5.959575 j
Therefore Ep = 6.0 J (2sf)