Answer
$9x^{3}-4.9x^{2}+11.1$
Work Step by Step
$ \begin{array}{rrrrrr}
& 1.2x^{3} & -3x^{2} & +9.1\\
+ & 7.8x^{3} & -3.1x^{2} & +8\\
+ & & 1.2x^{2} & -6\\
\hline & & & \\
= & 9.0x^{3} &+(-6.1+1.2)x^{2} & +17.1-6\\
& & & \\
= & 9.0x^{3} & -4.9x^{2} & +11.1\\
& & & \\
& & &
\end{array} $
