Find the rate of change of the width, diagonal and perimeter of a rectangle with constant area

Solution for:

A rectangle maintains a constant area of 300 cm?, even though its length and width are changing. Suppose that the length is increasing at a rate of 6 cm/sec.
(a) Find the rate at which the width is changing at the moment when the width is 15 cm.
(b) Find the rate at which the perimeter is changing at that same moment.
(c) Find the rate at which the length of the diagonal is changing at that same moment.