Hey, I have this problem that I am halfway done, but I do not know how to solve the second portion.

Question

Ship S is 41 Km due North of ship T. Ship S sails south at a rate of 9 km/h. Ship T sails west at 10 km/h.

a) At what rate are they approaching or separating one hour later?

b) When do they cease to approach one another and how far apart are they at this time?

Solution

a)

I figured out the distance after 1 hour: 33.52km

I then figured out the rate of change of the seperation/approach: 0.57kmh (approaching)

b) I do not know how to solve this part though. I could use the equation I used to find the rate of change, but setting that to zero really doesn't help in finding 3 other variables. Help?