On the Format Trendline Label pane that appears go to the Label Options tab. Then create this formula in say A18. Two dialog boxes direct you to first select the destination cell for the formula and second select the cell for the independent variable The utility converts a trendline equation such as y 3x3 25x2 - 5x -11.
Select the scatter chart and then click the Add Chart Element Trendline More Trendline Options on the Design tab.
On the Format Trendline Label pane that appears go to the Label Options tab. On the left is the most likely chart you will get. Depending on the degree of your polynomial trendline use one of the following sets of formulas to get the constants. Within the Charts group click on the plot option called Scatter.