You have the following function for the profit for selling x units of a product:
f(x) = 15x - 600
in order to determine the profit for 15,600 units, replace x = 15,600 into the previous function and simplify:
f(15,600) = 15(15,600) - 600 = 233,400
Hence, the profit for 15,600 units is $233,400
