At noon, ship A is 20 nautical miles due west of ship B. Ship A is sailing west at 21 knots and ship B is sailing north at 18 knots. How fast (in knots) is the distance between the ships changing at 4 PM? (Note: 1 knot is a speed of 1 nautical mile per hour.)

I took the derivitive of both sides dx/dt = 21 and dy/dt =18

and at 4pm, x= 20 +21*4, y=18*4

21+21+21+21+20 =104

18*4= 72

D = sqrt(x^2+y^2)

D' = 1/2(x^2+Y^2)^-1/2 *(2xx' +2xY')

Z= sqrt((104+72) + 20^2) = sqrt(31376)

104+72/sqrt(31376)*(21+18)

would you let me know if I am on the right path.

Thankx

keith