The wording is somewhat ambiguos. To avoid ambigüity, it should be stated to which frame of reference is reported the speed.of the basketball.
The options are:
1) If the speed of the ball (20 km/h) is refered to Diego, given tha the hoop is at rest, the spped of the ball relative to the net will be the speed of Diego + the speed of the ball: 7 km/h + 20 km/h = 27 km/h.
2) If the speed of the ball (20 km/h) is relative to the ground then, given that the hoop is fixed, the speed of the ball relative to the net is 20km/h.
