1. First, create a scatter chart with your data points and trendline. To do this, select your data and go to the Insert tab on the ribbon. Click on Scatter and choose the type of scatter chart you want to create.
2. Once you have your scatter chart, right-click on the trendline and select Format Trendline from the drop-down menu.
3. In the Format Trendline pane that appears on the right-hand side of the screen, select the Options tab.
4. Under Options, check the box next to Display equation on chart and Display R-squared value on chart. This will show the equation of the trendline on the chart.
5. Next, click on the Number dropdown menu and select Scientific. This will format the equation in scientific notation.
6. Now, you need to calculate the intercepts of the trendline with the x and y axis. To do this, you can use the equation displayed on the chart. The y-intercept is the point where the trendline crosses the y-axis, so you can set x = 0 in the equation to find the y-intercept. The x-intercept is the point where the trendline crosses the x-axis, so you can set y = 0 in the equation to find the x-intercept.
7. Once you have calculated the intercepts, you can add them to the chart. To do this, click on the Chart Elements button (the plus sign icon) on the top-right corner of the chart. Select Axis Titles and then Primary Horizontal Axis Title and Primary Vertical Axis Title. Enter the intercept values as the axis titles.
8. Finally, to extend the trendline to cut the x and y axis, you can manually adjust the axis scales. Right-click on the x-axis and select Format Axis from the drop-down menu. Under Axis Options, set the minimum value to the x-intercept you calculated earlier. Repeat this process for the y-axis, setting the minimum value to the y-intercept you calculated.

That's it! Your trendline should now extend to cut both the x and y axis on your scatter chart in Excel.