You can simply expand each product and see whether it gives you a difference of squares.
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That's actually
which is not a difference of squares.
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</span>•
which is not a difference of squares.
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That is a difference of two squares.
</span>————<span>—
</span>•
</span>which is not a difference of squares.
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Only the third option will result in a difference of squares.
Answer: (− 7x + 4) · (− 7x − 4).
I hope this helps. =)