Đề bài

Giải các hệ phương trình

a) $\left\{ {\begin{array}{*{20}{c}}{3x + y = 3}\\{2x - y = 7}\end{array}} \right.$

b) $\left\{ {\begin{array}{*{20}{c}}{x - y = 3}\\{3x - 4y = 2}\end{array}} \right.$

c) $\left\{ {\begin{array}{*{20}{c}}{4x + 5y = - 2}\\{2x - y = - 8}\end{array}} \right.$

d) $\left\{ {\begin{array}{*{20}{c}}{3x + y = 3}\\{ - 3y = 5}\end{array}} \right.$