Question

(x-3)(x^2+3x+9)-x(x^2-16)=21

Answer 1

$(x-3)(x^2+3x+9)-x(x^2-16)=21$

$x^3-27-x(x^2-16)=21$

$x^3-27-x^3+16x-21=0$

$16x-48=0$

$16x=48$

$x=48:16$

$x=3$

Ответ:  $3$

P. S.
$(a-b)(a^2+ab+b^2)=a^3-b^3$