the question is

A ship K is sailing due north at 16km/h, and a second ship R , which is 44 km north of K, is sailing due east at 10 km/h. At what rate is the distance between K and R changing 90 minutes later? Are they approaching one another or separating at this time? explain

so the two known rates are $\displaystyle \frac{dK}{dt} = 16, \frac{dR}{dt} = 10$ and the unknown is $\displaystyle \frac{ds}{dt}$

I am a little lost of what equation to use to relate them.

can I use $\displaystyle x^2 + y^2 = s^2$ ?