Sorted,
Did some digging around here, came up with x = (y-intercept)/slope
This seems to be working.
Thanks all.

"AndyB" wrote:

I have a graph which draws a line Rotations vs Time.
I want to make a cell where I can enter any amount of rotations and then
return the time in to another cell. Problem is it's not an easy relationship
between rotation and time.
Data:
Revs Time
5 20
10 40
15 55
20 70
25 85

If somone inputs 12 revs, the result should be 45