If he makes a base salary of 1100$, then Max's total salary is 1100+<some number>. That <some number> is the amount he makes in commissions, which is 7.5% (or 0.075 by moving the decimal 2 spots to the left). Multiplying 43000 by 0.075 to get his commision money, we get 3225. Remember that we still have to add 1100 to it (as that's his base salary), so we end up with 1100+3225=4325
