For finding the inflection points you'll have to put the first derivative equal to zero and solve for x. After you've done that, you can plug in the x you've just found into the original function and find also the y, so you'll find the ordered pair (x,y) of the inflection point(s).
5% is the same as .05, so in this case, the sales tax is added by .05. Subtract .05 from 24.45 to get the original price of the futon. You'll get $24.40
