Answer
$a.\quad 49x^{2}-36$
$ b.\quad (7x+6)(7x-6)$
Work Step by Step
$a.$
From the square with side of length $7x$, subtract 4 squares with sides of length $3$.
$(7x)^{2}-4\cdot 3^{2}=49x^{2}-36$
$b.$
$49x^{2}-36$= ... a difference of squares, $(7x)^{2}-(6)^{2}$
$=(7x+6)(7x-6)$
