gravitational potential energy : P
Gravity : g
Mass : m
height : h
P = mgh = 3 x 9.8 x 0.45 = 13.23 Joule
Potential energy is work , from the known formula
W = Fd ( work = force x distance )
W = P ( in case of potential energy height change)
F is the force acting on the body in case of ideal ramp , the only force acting is the weight of the body
F = mg ( not just <m> as the force is mg (Newton) gravity effect)
d is the displacement in direction of force, as we have considered the force to be the weight not it's component in direction of the ramp , the change in displacement is the change in height so
d = h
W = Fd = (F = mg) x (d = h) = mgh
W = mgh = P