$\frac{?}{{?}_{\frac{?}{?}}}3\mathrm{.}9$ Solve the following problems by the two$-$phase simplex method:

a$.$

maximize $3x_1+x_2$

subject $to{x}_1-x_2-1$

$-x_1-x_2-3$

$2x_1+x_{2}4$

$x_1,x_{2}0$

b$.$

maximize $3x_1+x_2$

subject $to{x}_1-x_2-1$

$-x_1-x_2-3$

$2x_1+x_{2}2$

$x_1,x_{2}0$

c$.$ maximize

subject to

$3x_1+x_2$

$x_1-x_2-1$

$-x_1-x_2-3$

$2x_1-x_{2}2$

$x_1,x_{2}0.$