Answer
a. $49x^{2}-36$
b. $(7x+6)(7x-6)$
Work Step by Step
a. Area of the shaded region = (Area of the square whose side is $7x$) - $4 \times $(Area of the square whose side is 3)
$=(7x)^{2} - 4 (3)^{2}$
$=(7x)^{2} - 4 (9)$
$=49x^{2} - 36$
b. In factored form
To factor the difference of the squares, use the formula
$(a^{2}-b^{2}) = (a+b)(a-b)$
$49x^{2} -36 = (7x)^{2} - 6^{2} = (7x+6)(7x-6)$
