Answer
The classmate forgot to multiply $2$ by the terms of the expression ($2x$ and $1$) and add that sum to their initial answer of $4x^2+1$.
Work Step by Step
$(2x+1)^2$
$(2x+1)(2x+1)$
$2x*2x+1*2x+1*2x+1*1$
$4x^2+2x+2x+1$
$4x^2+4x+1$
Algebra: A Combined Approach (4th Edition)
by Martin-Gay, Elayn
Published by
Pearson
ISBN 10:
0321726391
ISBN 13:
978-0-32172-639-1
Chapter 5 - Section 5.6 - Special Products - Exercise Set - Page 389: 101
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