Thinking in terms of graphs is useful but only takes you so far. The point is how to think about the data and/or functions underlying the graph.
You are looking for "interpolation" strategies.
Since both x and y look well behaved, you can achieve what you want simply by
x = [0 10 20 30 40 50 60 70 80]
y = [0 3 6 9 12 15 18 21 24]
yTarget = 8
xTarget = interp1(y,x,yTarget,"linear")
In fact, upon trying above and actually looking at your data, you have labored to define a very simple line, which by inspection has the form
So for your specific problem just do algebra
But anyway the procedure in the code above will work well as long as y is monotonic.