After charting your data select the "add trend" line option. You are given the choice of several different line fits, choose the one that is defined by the under lying math of your data... Or chose the one that is a close match to the trend in your data. In the trendlines options there is a check box for displaying the equation on the chart.
If your data is linear, you can also use the LINEST command to compute the slope and intercept of the best fit line. This is the same as choosing a linear fit trend line, except the slope and intercept are more easily used to extrapolate (this the official name of extending a trend line beyond your data set.
You can also extrapolate by selecting your data and "drag fill" the column until you get beyond the x value you need. For this to work you need a constant [itex] \Delta [/itex]x in your data.
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