the length of a rectangle is given by 6t+ 5 and its height is $\displaystyle sqrt(t)$, where t is time in seconds and the dimensions are in centimeters. Find the rate of change with respect to time.

I am not really quite sure how to do rate of change.. I have read over the examples given for rate of change in the book however, it seems vague and borderline unrelated.. Their problem was dealing with speed and velocity..