The problem statement tells us ...
... paycheck = 4000 + 0.09×sales
and that the paycheck amount is 11650. Substituting that into the above equation and solving for sales, we have ...
... 11650 = 4000 + 0.09×sales
... 7650 = 0.09×sales . . . . . . . . . . subract 4000
... 7650/0.09 = sales = 85000 . . . divide by 0.09
John's sales during that month were $85,000.
