Put the following problems into standard form.

$($a$)$

$]7[0$

$(b)\{\begin{array}{ccc}\text{:}[& \text{M}\text{i}\text{n}\text{i}\text{m}\text{i}\text{z}\text{e}& x_2+x_3+x_4]\\ & \text{s}\text{u}\text{b}\text{j}\text{e}\text{c}\text{t}to& 6\\ x_1+x_{2}1\\ x_2+x_3-x_{4}1\\ 5x_1-6x_2+7x_3-8x_{4}2\\ x_{1}0,& x_{2}0,x_3,x_4\text{u}\text{n}\text{r}\text{e}\text{s}\text{t}\text{r}\text{i}\text{c}\text{t}\text{e}\text{d}& \end{array}$