The steps are the following. You need to find Dickie's walking speed.
You know "time travelled", so you only need to find "distance travelled."
You can apply Pythagoras' theorem to find the distance from D to A if you know AB and BD.
AB is given (350 meters), and BD = CD + 300 meters, so you need to find CD, which you can derive from the fact that CE as well as the angle at D are given:
so , and .
so the speed is meters per minute. (Convert to whatever unit required.)
Try to understand the steps above, and then solve problem (b) on your own.