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 and the unknown is

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

can I use ?