## College Algebra (10th Edition)

According to the Binomial Theorem we can obtain the $r+1$th term of the expansion of the binomial $(x+y)^n$ by the formula $_nC_rx^{n-r}y^r$. Hence here it is: (by plugging in $n=7,r=5$ and $2x,1$: $_7C_5(2x)^{7-5}1^5=21\cdot 4x^2\cdot1=84x^2$. Thus the coefficient is $84$.