Answer
$x^3+21x^2+147x+343$
Work Step by Step
$(x+7)^3$
$(x+7)(x+7)(x+7)$
$[(x+7)(x+7)](x+7)$
$[x(x+7)+7(x+7)](x+7)$
$[x^2+7x+7x+49](x+7)$
$(x^2+14x+49)(x+7)$
$x(x^2+14x+49)+7(x^2+14x+49)$
$(x^3+14x^2+49x)+(7x^2+98x+343)$
$x^3+21x^2+147x+343$