I am having big problems with the second part of this question.

a) find a positive integer k such that

$\displaystyle

(x^2)^{20-k}(\frac{1}{x})^k=x

$

I have done this using the rules of powers and get k=13

b) Using the binomial theorem and the answer from a) determine the coefficient of x in the expansion of.

$\displaystyle

(2x^2-\frac{1}{x})^{20}

$

I don't see how the answer from part a) can help