Originally Posted by

**jvignacio** hey guys, question:

Find the coefficient of $\displaystyle x^{-4}$ in $\displaystyle (4x^7-2x^{-3})^8$

I know we are ment to rewrite $\displaystyle (4x^7-2x^{-3})^8$ so there are two expressions to the power of 8 but im not sure how we are ment to rewrite it. I got the answer $\displaystyle (x^{-3})^8 (4x^{10} -2)^8$ but not sure how they got this... then it says we see that finding the coefficient of $\displaystyle x^{-4}$ is equivalent to finding the coefficient of $\displaystyle x^{20}$.. how did they get $\displaystyle x^{20}$ ?

thank you..