#### Answer

A

#### Work Step by Step

$3n^2+10n+3$ is the area
one side is $n+3$
$(n+3)(an+b)$
$an^2+bn+3an+3b$
$an^2+bn+3an+3b = 3n^2+10n+3$
$an^2=3n^2$
$an^2/n^2=3n^2/n^2$
$a=3$
$an^2+bn+3an+3b = 3n^2+10n+3$
$3n^2+bn+3*3n+3b = 3n^2+10n+3$
$3n^2+bn+9n+3b = 3n^2+10n+3$
$3n^2+bn+9n+3b = 3n^2+10n+3$
$bn+9n+3b = 10n+3$
$(b+9)n+3b=10n+3$
$b+9=10$
$b=1$
$an+b=3n+1$