Since gravitation is a conservative force, the work done in moving a body from one point to another in the gravitational field depends only on the end points. It does not depend on the path between the end points. It equals the negative of the change in gravitational potential energy.
U(h) = mah, where a = gravitational acceleration on the planet
At h = 20 m, U/m = 16 J/kg
U/m = ah ⇒ a = (U/m) / h = (16 J/kg) / (20 m) = 0.80 m/s2
U1 = gravitational potential energy at height h1
h1 is ground level = 0 m
U1 = ma(0) = 0
U2 = gravitational potential energy at height h2
h2 = 8 sin(60º) m = 6.93 m
W1→2 = -(U2 - U1) = U1 - U2 = 0 - U2 = -mah2
W1→2 = -(4 kg)(0.80 m/s2)(6.93 m) J = -22.2 J
The negative sign means work was done against the gravitational force.