2 cars move from same point. one travels north 28mi/hr and the other travels east 70mi/hr

a) write an equation to relate these variables

i did dz/dt = x*dx/dt + y*dy/dt * 1/z

b) at what rate is the distance between the cars increasing 5 hours later?

so z^2 = x^2 + y^2

5*28 = 140

5*70 = 350

z = sqrt(142100)

dz/dt = (140*28 + 350*70)/ 376.96

dz/dt ~= 75.39

did i work it out and calculate correctly?

the problems i'm working w/ don't have an answer key and i'm wondering if this is the right answer as well as proper process to go about a problem like this