ok well ill give a simple problem, to demonstrate my problem properly :p

say you have 2x+3=y

you stick that in the calculator as y1=2x+3

you press F2, and go to zoomfit.

ok here's the problem: i know you can go to F5 from here and go to value, and type in an x coordinate and the calc will give you the y coordinate. But how do you do this the other way around? and put in the y-coordinate, and get the x coordinate?