Answer
$16-9x^2$
Note that standard form would list the variables first in decreasing order of exponent, so another way to write this answer is: $-9x^2+16$.
Work Step by Step
$$(4-3x)(4+3x)$$ $$=(4\times 4)+(4\times 3x)+(-3x\times 4)+(-3x\times 3x)$$ $$=16+12x-12x-9x^2$$ $$=16-9x^2$$
Note that standard form would list the variables first in decreasing order of exponent, so another way to write this answer is: $$-9x^2+16$$
