Finding the Y coordinate with the X coordinate

OK so this is a bit confusing to explain for me, so I drew a little diagram:

http://img269.imageshack.us/img269/1104/problembvd.png

Say we have someone walking up a hill, but only know:

-Their X coordinate

-The X/Y of the bottom of the hill

-The X/Y of the top of the hill

Could someone give me a little equation to find their Y coordinate?

Sorry if this is painfully obvious, but I do not have a very good math background. Also, sorry if this is in the wrong forum.