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As Media has mentioned values of A,b,c are passed through those matrices.
To understand it better I'm naming the constraints from top to bottom i.e., 1st constraint as constraints-1,......constraint-4.
Firstly, lets talk about matrix b which consists of all the values on the right hand side of the constraints i.e., 1,2,0,4. He converted all the constraints to $⩽$ by multiplying with -1 on both sides. So he multiplied with -1 for constraints-2 and 3.
Now the constraints-2 has changed from $x_1 + x_2 ⩾ 2$ to $-x_1 - x_2 ⩽ -2$
Values of Matrix b would be 1,-2,0(-0),4
Now lets go through the constraints i.e.,
- $-x_1 + x_2 ⩽ 1 $ for the coefficient of $x_1,x_2$ are -1 and 1. The values of Matrix A row 1 values are -1,1
- Similarly for row 2 to row 4 as you have 4 contraints.
Finally our objective function coefficients, are the values of c i.e., 2 and 1.
Do let me know if you have any more doubts.