I make the equations p0 and p1 equal to find y, and use the solve function, but always remind me of the following errors. How can I figure out y? Q in the formula is equivalent to a constant. Thank you for your answer!

Here is my program：

question.mw

I would like to place a legend between the axes and the image, but there is no such place in the positions given by the software. How to do it?