Simplify (3x^3+4x^2)+(3x^3-4x^2-9x)

$(3 x^{3} + 4 x^{2}) + (3 x^{3} - 4 x^{2} - 9 x)$

Remove parentheses.

$3 x^{3} + 4 x^{2} + 3 x^{3} - 4 x^{2} - 9 x$

Combine the opposite terms in $3 x^{3} + 4 x^{2} + 3 x^{3} - 4 x^{2} - 9 x$ .

Subtract $4 x^{2}$ from $4 x^{2}$ .

$3 x^{3} + 3 x^{3} + 0 - 9 x$

Add $3 x^{3} + 3 x^{3}$ and $0$ .

$3 x^{3} + 3 x^{3} - 9 x$

Add $3 x^{3}$ and $3 x^{3}$ .

$6 x^{3} - 9 x$