D2 Edexcel: Game Theory Question

Hey, I'm revising for a D2 exam tomorrow (I know, left it rather late) and am fine apart from one thing; and that's as follows:

When you are working out mixed strategies for a 3x2 game graphically, you plot the graph, and then find the location of a point of intersection of two of the lines. What I don't understand is how you work out which point of intersection to use.

That's probably a bit hard to understand, so I'll try and phrase it more simplistically!

If we get given a tableau as below:

And are asked to work out the optimal mixed strategy for X, I can get to the following graph:

But I do not understand why we use point P as our value, rather than Q or R (which I have marked on as 'alternatives').

Any help is much appreciated!!