How do you multiply (9−2x4)2?

Explanation:
${\displaystyle {(9-2x^4)}^2}$
${\displaystyle =(9-2x^4)(9-2x^4)}$
${\displaystyle =81-18x^4-18x^4+4x^8}$
${\displaystyle =81-36x^4+4x^8}$

Multiply/Simplify/Expand:
${\displaystyle {(9-2x^4)}^2}$
Use the square of a difference:
${\displaystyle {(a-b)}^2=a^2-2ab+b^2}$,
where:
a=9, and ${\displaystyle b=2x^4}$
Plug in the known values.
${\displaystyle {(9-2x^4)}^2=9^2-(2\cdot 9\cdot 2x^4)+{\left(2x^4\right)}^2}$
Simplify ${\displaystyle 9^2}$ to 81.
${\displaystyle {(9-2x^4)}^2=81-(2\cdot 9\cdot 2x^4)+{\left(2x^4\right)}^2}$
Apply the multiplicative distributive property: ${\displaystyle {\left(ab\right)}^m=a^m{b}^m}$
${\displaystyle {(9-2x^4)}^2=81-(2\cdot 9\cdot 2x^4)+2^2\cdot {\left(x^4\right)}^2}$
Apply power rule: ${\displaystyle {\left(a^m\right)}^n=a^{m\cdot n}}$
${\displaystyle {(9-2x^4)}^2=81-(2\cdot 9\cdot 2x^4)+2^2\cdot x^8}$
Simplify.
${\displaystyle {(9-2x^4)}^2=81-36x^4+4x^8}$
Rearrange the equation in descending order.
${\displaystyle {(9-2x^4)}^2=4x^8-36x^4+81}$