## Precalculus (10th Edition)

$84$
According to the Binomial Theorem, the term containing $x^j$ in the expansion of $(ax+b)^n$ is given by: ${n\choose n-j}a^jb^{n-j}x^j.$ Hence, the coefficient of $x^2$ is ${7\choose 7-2}2^2(1)^{7-2}=84$