∴y=-ax+P.

Therefore, the objective function value z is the intercept of the straight line family y=-ax+P,

When the slope of the straight line family y=-ax+P is equal to the slope of the boundary AC,

There are countless optimal solutions to obtain the maximum value of the objective function z=ax+y,

At this point, -a=5? 3 1? 5=- 12,

That is a= 12,

So choose a.