## Algebra 1: Common Core (15th Edition)

The middle term should be $-2(21a)$. Result: $9a^{2}-42a+49$.
The middle term should be $-2(21a)$because $(a-b)^{2}=a^{2}\color{red}{-2ab}+b^{2}$ ...so, $(3a-7)^{2}=9a^{2}-42a+49$.