What's the square root of 34 rounded to the nearest tenth?

You take the discount first. Alex is completely right. Laura is if and only if that 110% is really just 10%.  So I'll use Alex's method because it is for certain correct.

28.5*35/100 = 9.975 (which is the amount of the discount)

28.5 - 9.975 = 18.525 (which is the amount paid with the discount factored in.)

Now you take 10% of this amount and add it on which gives 18.525 * 10/100=1.825 which is the amount of the tax.

Add this on to the selling price 18.525 + 1.825 = 20.3775 and now is the time to do your rounding.

The cash register will ring up 20.38 which is just under $21.00

$20.38