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,
$(3a-7)^{2}=9a^{2}-42a+49$.
Algebra 1: Common Core (15th Edition)
by Charles, Randall I.
Published by
Prentice Hall
ISBN 10:
0133281140
ISBN 13:
978-0-13328-114-9
Chapter 8 - Polynomials and Factoring - 8-4 Multiplying Soecial Cases - Practice and Problem-Solving Exercises - Page 508: 50
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