Homework Question for the weekend!

Solve the following Question by Monday, April 7^th:

Joe and Dave have planned to meet for coffee at a certain intersection (by Starbucks). Joe is travelling east toward the intersection at a speed of 60 km/h, and Dave is travelling south toward the intersection at a speed of 50 km/h. How fast is the distance between them changing when Joe is 0.4 km from the intersection, and Dave is 0.3 km from the intersection?