Related rates

Problem

A sailboat 3 miles west of Point Stuart sails toward Point Stuart at 4 miles per hour. At the same moment, a motorboat 4 miles north of Point Stuart travels due south toward Point Stuart at 12 miles per hour.
At that moment in time, what is the rate of change in the distance between the sailboat and the motorboat in miles per hour?
space
space, start fraction, m, i, l, e, s, divided by, h, o, u, r, end fraction