Test analysis: (1) Substitute x=2 and y=- 1 into the equation to calculate the value of A;

(2) List all equally possible case numbers, find out that the case number randomly selected by A and B is exactly the solution of the equation ax-y=5, and then we can get the probability.

Problem analysis: (1) Substitute x=2 and y=- 1 into the equation: 2a+ 1=5, that is, A = 2；;

(2) List:

?

2

three

-5

( 0 , -5? )

( 2 ,-5)

( 3 , -5? )

- 1

( 0 ,? - 1 )

( 2 ,- 1)

( 3 , - 1? )

1

( 0 ,? 1 )

( 2 , 1? )

( 3 , 1? )

?

There are nine equal possibilities, in which (x, y) happens to be the solution of equation 2x-y=5, and there are three cases (0, -5), (2,-1), (3, 1) and * *, then P= =