Well, it seems that I haven't got my question across.
On the chart, there is the x-y equation which is automatically generated in making the scatter chart.

However, we cannot take out the x-y equation. If I want to use it in a cell, I have to manually type the x-y equation provided on the chart (e.g. y = 0.548x + 1.369) in that cell.

But anyway, it seems that I've found a solution. In that cell, I manually type an equation:

y = mx + b
where m = slope and b = interception

and that equation is the same to that generated from the scatter chart.

However, a question is still unanswered. That is - how can I get the interception between the markers and the trendline on the scatter chart?