Maximize following function using Simplex method;

$f=x_1+3x_2$

subject $to$

$-x_1+x_2+x_3=1$

$x_1+x_2+x_4=2$

$x_{i}0$ where $i=1,2,3,4$

Where $x_1,x_2,x_3,x_4$ are slack variables