Please follow the steps below to add a Trendline to a Chart:
Step 1: Click anywhere on the chart, the "Chart Design" Tab (or Design Tab if you have a different version) will appear in the Ribbon;
Step 2: Click the "Chart Design" Tab from the ribbon;
Step 3: Click "Add Chart Element" in the Chart Layouts area;
Step 4: Click "Trendline" from the drop down list, then select one of the following Trendline types "Linear", "Exponential", "Linear Forecast" or "Moving Average";
Please note that the "Trendline" will be removed when select "None" from the list.
Step 5: The following is the example of the exponential Trendline;
Step 6: To display trendline equation: right click on the trendline, select "Format Trendline" from the dialog box, then check "Display Equation on chart".
To delete "Trendline", the easy way is to click on it and press "Delete" key in the keyboard, or repeat the step 1 to step 3, then select "None" from the drop down list.