$\{\begin{array}{ll}5x-y=4& \\ -2x+y=5& \end{array}$
5x − y − 2x + y = 4 + 5
3x = 9
x = 3
5x − y = 4
y = 5x − 4 = 5 * 3 − 4 = 15 − 4 = 11
Ответ: (3; 11)

$\{\begin{array}{ll}3x+5y=10& \\ 3x-7y=4|\ast (-1)& \end{array}$
$\{\begin{array}{ll}3x+5y=10& \\ -3x+7y=-4& \end{array}$
3x + 5y − 3x + 7y = 10 − 4
12y = 6
y = 0,5
3x + 5y = 10
3x = 10 − 5y
3x = 10 − 5 * 0,5
3x = 10 − 2,5
3x = 7,5
x = 2,5
Ответ: (2,5; 0,5)

$\{\begin{array}{ll}x+4y=-7& \\ x-9y=6|\ast (-1)& \end{array}$
$\{\begin{array}{ll}x+4y=-7& \\ -x+9y=-6& \end{array}$
x + 4y − x + 9y = −7 − 6
13y = −13
y = −1
x + 4y = −7
x = −7 − 4y = −7 − 4 * (−1) = −7 + 4 = −3
Ответ: (−3; −1)

$\{\begin{array}{ll}3x-4y=-5& \\ 6x+4y=-1& \end{array}$
3x − 4y + 6x + 4y = −5 − 1
9x = −6
$x=-\frac{6}{9}=-\frac{2}{3}$
3x − 4y = −5
$3\ast (-\frac{2}{3})-4y=-5$
−2 − 4y = −5
−4y = −5 + 2
−4y = −3
$y=\frac{3}{4}$
Ответ: $(-\frac{2}{3};\frac{3}{4})$