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 .
Now ,
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.