Question

a thin uniform rod of length l cm and a small particle lie on a line separared by a distance of a cm. if K is a positive constant and F is measured in newtons, the gravitational force between them is F=K/(a(a +l))
a) if a is increasing at the rate 2cm/min when a=15 and l=5, how fast is F increasing?
b)if l is decreasing at the rate 2cm/min when a =15 and l=5, how fast is F increasing?