#### Answer

$49x^2+42xy+9y^2$

#### Work Step by Step

$\bf{\text{Solution Outline:}}$
Use the special product on squaring binomials to simplify the given expression, $
(7x+3y)^2
.$
$\bf{\text{Solution Details:}}$
Using the square of a binomial which is given by $(a+b)^2=a^2+2ab+b^2$ or by $(a-b)^2=a^2-2ab+b^2,$ the expression above is equivalent to
\begin{array}{l}\require{cancel}
(7x)^2+2(7x)(3y)+(3y)^2
\\\\=
49x^2+42xy+9y^2
.\end{array}