Question 123644
First, one small correction!
This is not an "equation" but an "expression" because there is no = sign.
In an equation, you would have something on the left side equal to something on the right side. 
Ok, Add:
{{{(6+3x+9x^2+8x^3) + (4-3x+9x^2-8x^3)}}} First, group the like-term:
{{{(8x^3-8x^3)+(9x^2+9x^2)+(3x-3x)+(6+4)}}} Perform the indicated addition/subtraction:
{{{(0)+(18x^2)+(0)+(10)}}} Finally...
{{{18x^2+10}}}