We set up the equation given the conditions listed. Since it says 5 times the difference between a number and 7, we can use x as a placeholder for the number.
(x-7) is the difference between the number and 7. The question asks for 5 times that difference thus we multiply the entire thing by 5 giving us 5(x-7. The question says that it all equals -15 so we set that expression equal to -15.
