D $3\mathrm{.}1-4.*$ Use the graphical method to solve the problem:

Maximize $Z=2x_1+x_2,$

subject to

$x_{2}10$

$2x_1+5x_{2}60$

$x_1+x_{2}18$

$3x_1+x_{2}44$

and

$x_{1}0,x_{2}0.$