$(16x^4)(16x^4)$

$256x^8$

$16x^8$

$(2x^2)(2x^2)(2x^2)(2x^2)$

$16x^8$

$(4x^4)(4x^4)$

$16x^8$

$(2x^4)(2x^4)(2x^4)(2x^4)$

A negative exponent means that the fraction and the variable must be reciprocated (flipped).