Answer
$(4z^2+5)(z^2-3)$
Work Step by Step
Step 1. Let $z^2=y$, we can rewrite the expression as $4z^4-7z^2-15=4y^2-7y-15$
Step 2. $4y^2-7y-15=(4y+5)(y-3)$
Step 3. Use $y=z^2$, we have $4z^4-7z^2-15=(4z^2+5)(z^2-3)$
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