Question 246029
OK, the east-west displacement (in miles) between the cars at time t (in hours) is 60t. The north-south displacement is 35t. So the distance is {{{sqrt((60t)^2 + (35t)^2)}}} by Pythagoras; that works out to be 65t. 

The rate of change of the distance is the derivative of that, or 65 miles per hour.