Intersection of lines?

[amca01]

I'm very much a newbie here, but my needs are simple. I have a function f(x) which I can plot, and given x values a and b, say, I draw the line between (a,f(a)) and (b, f(b)). Given two such intersecting lines, I need to find their intersection, and put a mark and label at it.

I know that intersections can be done in "plain" Cetz, but can they be done within a plot construct in Cetz-plot? Many thanks.