2)

[tex]\left \{ {{5x - 2y = 12 \: \: | \times ( - 3) } \atop {15x - 3y = - 3 \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: }} \right. \\ \\ \left \{ {{ - 15x + 10y = - 36} \atop {15x - 3y = - 3}} \right. + \\ 7y = - 39 \\ y = - 5 \frac{4}{7} [/tex]

И вместо у поставим -5 4/7

[tex]5x - 2y = 12 \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \\ 5x - 2 \times ( - \frac{39}{7} ) = 12 \\ 5x + \frac{78}{7} =12 \\ \frac{35x + 78}{7} = 12 \\ 35x + 78 = 84 \\ 35x = 6 \\ x = \frac{6}{35} [/tex]

5)

[tex]\left \{ {{10x - 9y = 8} \: \: \: \: \: \: \: \: | \times - 3 \: \: \: \atop {21y + 15x = 0.5 \: | \times 2}} \right. \\ \\ \left \{ {{ - 30x + 27y = - 24} \atop {42y + 30x = 1}} \right. + \\ 63y = - 23 \\ y = - 2 \frac{17}{63} [/tex]

И вместо у поставим -2 17/63

[tex]10x - 9y = 8 \\ 10x - 9 \times ( - \frac{23}{63} ) = 8 \\ 10x + \frac{23}{7} = 8 \\ \frac{70x + 23}{7} = 8 \\ 70x + 23 = 56 \\ 70x = 33 \\ x = \frac{33}{70} [/tex]