Hello,
Yes a plot helps, but in this case you did not say how it helps. For example, are you saying that there are an infinite number fo solutions or just one? In other words, you need to state what your final solution to this problem would be.
We both know what it is, but someone else might not know what you are implying with the graph.
BTW it's a little clearer to graph y=f(x)-y1 rather than y=f(x).
Back to main ideas...
The derivative of the inverse function can be found using the property that the inverse function slope is 1/m where m is the slope of the original function. Watch the sign, and probably you want the derivative at the solution point.