A kitchen remodeling company determines that the marginal cost, in dollars per foot, of installing x feet of kitchen countertop is given by C'(x) =...

A kitchen remodeling company determines that the marginal cost, in dollars per foot, of installing x feet of

kitchen countertop is given by

C'(x)

= 7x^-1/3

.

Find the cost of installing an extra

13 feet of countertop after 30 feet have already been ordered.

this question is using the antiderivative and i as far as solving it to 21/2x^2/3 and then putting into the formula for the problem which is

21/2(43)^2/3-21/2(30)^2/3 which is suppose to equal 27.5 but i got something else what do you think i did wrong ?

The bold is what i got.