Power, measured in watts, is the amount of time it takes for a given amount of work to be done. The faster it takes something to perform a given amount of work, the more power it has generated. Since there are two things performing work on the ball, these two things are also producing power. To find the power from both the racquet and gravity, we simply take the amount of work done by each and divide it by the amount of time the force was applied for:
Therefore the power generated by gravity was -0.34 W.
Therefore the power generated by the racquet was 5.8x10^3 W.