Data Sufficiency

Statement 1
Since the salesman sold 5 units in this scenario, his performance bonus will be given by ax^2+ax. His bonus is $300, so ax^2+ax = 300 when x = 5. By plugging in 5 for x, we should be able to solve for a.

If we know a, we can plug it into our equation ax^2+ax and set x equal to 10 to solve for the salesman's bonus for the first 10 units. Then, we can plug a into the equation for selling more than 10 units (ab^2+50b).

Then, since 28 units is 18 additional units past 10 units, we'll set b equal to 18 and solve for the salesman's additional bonus.

Finally, we can add the bonus for the first 10 units to the bonus for the additional 18 units to get the salesman's total bonus. We don't actually need to do any of these calculations (and we shouldn't because it's a huge waste of time) - we only need to know that we can. Sufficient.

Statement 2
Since the salesman sold 15 units in this scenario, he would have sold 5 additional units, which means b = 5, and his additional performance bonus will be given by ab^2+50b. His additional bonus is $500, so ab^2+50b = 500 when b = 5. By plugging in 5 for b, we should be able to solve for a.

Once we have a, we can do the exact same process we did with Statement 1 to solve for the salesman's total bonus. Again, we don't actually need to do the calculations - knowing we could if we wanted to makes this statement sufficient.