ANSWER
B.
[tex]7( {a}^{2} + 2)( {a}^{2} - 2)[/tex]
EXPLANATION
The given expression is
[tex]7 {a}^{4} - 28[/tex]
First factor 7 to get,
[tex]7 ({a}^{4} - 4)[/tex]
Write the expression in the parenthesis as difference of two squares:
[tex]7 (({ {a}^{2} )}^{2} - 4)[/tex]
Recall that:
[tex]{x}^{2} - {y}^{2} = (x + y)(x - y)[/tex]
We apply this to our expression to get:
[tex]7 (({ {a}^{2} )}^{2} - {2}^{2} ) = 7( {a}^{2} + 2)( {a}^{2} - 2)[/tex]
