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