Answer
$16x^{2}-49y^{2}$
Work Step by Step
$(4x+7y)(4x-7y)$
...identify which terms correspond to $a $ and $b $
in the rule for the product of a sum and difference ($(a+b)(a-b)=a^{2}-b^{2}$).
$a=4x,\ b=7y$
...substitute for $a $and $b $ in the rule
$(4x+7y)(4x-7y)=(4x)^{2}-(7y)^{2}$
...simplify.
=$16x^{2}-49y^{2}$
