Find out x at y

I am trying to figure out a function or some script to determine at what point in the objects x-axis it crosses a point on the y-axis. That's not very well described so below is an image of what I mean.

I know the y value and the angle the object is at but this will continuously change, and so the point on the x-axis that it crosses y=1 will always change aswell, is there anyway of determining this value.