Attached is what you can do on excel.
Add the values as presented on the image. Select the values and then go t the menu "Insert" and look for "Charts" and then "Insert scatter chart". After this, select one of the dots from the graph and then press the right button of the mouse and select "add trendline". Now you have a trend line. Format the trendline selecting the option "Display Equation on chart" (and also if you want "Display R-squared value on chart"). Now you have the equation and you can easily predict the value when X is 15, you just need to substitute on the equation the X by the 15:
y = 4.0933Χ + 21.5 <=> y = 82.8995