Answer
$(a+7)(a-7)$
Work Step by Step
$a^{2}-49=$
...rewrite $49$ as a square.
$=a^{2}-7^{2}$
...factor using the rule for a difference of two squares.
($a^{2}-b^{2}=(a+b)(a-b),\ a=a,\ b=7$)
$=(a+7)(a-7)$
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