Interpolation problem I think?

[wenlianglo]

Hi there,
I am currently facing few problems with my report. The first thing is, how do I extrapolate a graph? I have a series of data from experiment which shows roughly a "mountain" like graph where there is ascending zone, peak zone, then descending zone. However, I didn't have enough value in the descending zone to plot a longer graph. How do I extrapolate?

Second question, is how to find X value. Imagine the "mountain" shaped graph. At one Y axis, there should be 2 X axis value since there an ascending and descending zone in the graph. Is there any function to help me find the 2 X value assuming I know what Y value to look at?

Thanks in advance.

I hope to add more details which hopefully does not confuse anyone. Assuming my graph have 15 X and Y value which made 15 points. Say, first 10 points is from ascending to peak zone of the graph while point 11 - 15 is in the descending zone. If I assumed my Y value to read the 2 X axis value is one of the first 10 points, say point 7, drawing a horizontal line to the descending zone, it's between point 12 and point 13. So, by theory, I should interpolate point 12 and point 13 to get the second X value from the Y axis starting at point 7. How do I write an equation to let excel do this automatically?

Thanks again