Help with this questtion it is 12th grade math. plz help

Alake near the arctic circle is covered by a sheet of ice during the cold winter months. when spring arrives, the ice starts to melt. s(t) models the ice sheet's thickness (in meters) as a function of time tt (in weeks). s(t)=-0.25t+4

Can someone me with my hwplz(only the circled ones)

What is the product? a-3/15a × 5/a-3