At 1 P.M., ship A is 150 km west of ship B. Ship A is sailing east at 35 km/h and ship B is sailing north at 25 km/h. How fast is the distance between the ships changing at 5 P.M.?

for this problem i first do is 35*4=140 and 25*4=100 so it mean the x is 10 and y is 100 and then take sqrt(10^2+100^2) this will find the c.
so once i have c i can do 2a(da/dx)+2b(db/dx)=2c(dc/dx)
and then i plug in the number
(1/sqrt(10^2+100^2))(10(35)+100(25))=(dc/dx)
so i find the answer is 28.35 but i don't think this is right? how do i know the x and y values?