Answer:
[tex]12x^2+8x-9[/tex]
Step-by-step explanation:
[tex]\begin{aligned}&\textsf{Given expression}: \quad & (3x+8x^2)+(4x^2+5x-9)\\&\textsf{Remove the parentheses}: \quad & 3x+8x^2+4x^2+5x-9\\&\textsf{Collect like terms}: & 8x^2+4x^2+3x+5x-9\\&\textsf{Combine like terms}: & 12x^2+8x-9\end{aligned}[/tex]
