The current revenue is 30*12 = $360

It can be said to be of the form xy = R

where x is the number of floor sanders rented, y the price per sander and R the revenue.

Also, we see that as y increase, x decreases, both linearly. We can say that y = -kx + c

First case, x = 30, y = 12.

12 = -k(30) + c

12 = -30k + c

Second case, x = 29, y = 14.

14 = -k(29) + c

14 = -29k + c

Using elimination;

2 = k

k = 2

Then, c becomes 72

So;

y = -2x + 72

Replace that in the initial equation;

x(-2x + 72) = R

simplify;

At R max, dR/dx = 0.

Solving for x gives x = 18.

When x = 18, y = 36.

Revenue becomes 18*36 = $648