Ihave a homework problem on functions, its a expondental problem how do i right a equation for it

Find the constant of variation for the relation and use it to write an equation for the statement. then solve the equation.

Let f(x) = 3 − x . find the average rate of change of f(x) from x = a to x = a + h and simplify your answer so that no single factor of h is left in the denominator.

Tom can paint the fence in 12 hours, but if he works together with a friend they can finish the job in 8 hours. how long would it take for his friend to paint this fence alone?