Separate area under a curve into two sections.
 

I have data that generates points where each X value is less than or equal to
the one before.

I can find the area under this curve determined by these points.

I want to know how to find the y-intercept for a line that is basically
horizontal, but becomes congruent with the line formed by the points above
after they intercect that is determined by a number representing the area
under it.

Here's my attempt to illustrate:

|
|
|.
| .
| .
| ...
| . I know these points
| .
| .
| .
| ......
| .
L|------------------...
| ...
|
| I know this area
| and the total area |
|
|________________________


I need the y-intercept of the line (L)
determined by the a particular area.

I hope this can be done without great dificulty in using Excel

Although I majored in math, and was quite good at it at one time, haven't
had occasion to do a calculus problem for the better part of 40 years.

Any help appreciated.