First you need to subtract the 8x from both sides to set the equation equal to 0.
enter the equation into the y= menu. You may not be able to see the graph for this because of the zoom settings. In this case go to the zoom menu and zoom out so you can see the entire graph.
then use 2nd calc and th zeros function.
set the left bound by moving the cursor to the left side of where the figure crosses the x axis and the same with the right bound.
Press enter. It should give you the answer.