Hello, everyone, first time to be here. Beginner of Mathematica.

I wonder how I can get the coordinates of certain points from a curve (may not be able to presented by equations). Anyway, let's go with a simple equation:

y==x^2

Now I calculate 4 values (x==1,2,3,4) to get y values. I can have 4 points and then I use listlineplot to get a curve. (I did not draw y==x^2 directly)

So now how can I extract certain point coordinate from this curve. Say I have y==25, how can I get the x value (without using the equation)?

And how can I show the point on the curve with dashed lines extended to x and y directions?

Thank you.