Answer
a) $(4+9x^2)(2-3x)(2+3x)$
b) Both 16 and $x^4$ are raised to the fourth power.
c) $(16x^4-81)$ is an example.
Work Step by Step
a)
$16-81x^4 = (4+9x^2)(4-9x^2) = (4+9x^2)(2-3x)(2+3x)$
b)
$16 = 2^4$
c)
$(16x^4-81) = (4x^2+9)(4x^2-9) = (4x^2+9)(2x+3)(2x-3)$
