Answer
$a.\quad 9x^{2}-16$
$ b.\quad (3x+4)(3x-4)$
Work Step by Step
$a.$
From the square with side $3x$, subtract 4 squares with sides 2.
$(3x)^{2}-4\cdot 2^{2}=9x^{2}-16$
$b.$
$9x^{2}-16$= ... a difference of squares, $(3x)^{2}-(4)^{2}$
$=(3x+4)(3x-4)$
