$\{\begin{array}{ll}x=2y-3& \\ 3x+4y=1& \end{array}$
3(2y − 3) + 4y = 1
6y − 9 + 4y = 1
10y = 1 + 9
10y = 10
y = 1
x = 2y − 3 = 2 * 1 − 3 = 2 − 3 = −1
Ответ: (−1; 1)

$\{\begin{array}{ll}8x-y=5& \\ -9x+2y=4& \end{array}$
$\{\begin{array}{ll}y=8x-5& \\ -9x+2y=4& \end{array}$
−9x + 2(8x − 5) = 4
−9x + 16x − 10 = 4
7x = 4 + 10
7x = 14
x = 2
y = 8x − 5 = 8 * 2 − 5 = 16 − 5 = 11
Ответ: (2; 11)

$\{\begin{array}{ll}2x-5y=21& \\ y=3x+1& \end{array}$
2x − 5(3x + 1) = 21
2x − 15x − 5 = 21
−13x = 21 + 5
−13x = 26
x = −2
y = 3x + 1 = 3 * (−2) + 1 = −6 + 1 = −5
Ответ: (−2; −5)

$\{\begin{array}{ll}x-5y=4& \\ 3x-8y=-2& \end{array}$
$\{\begin{array}{ll}x=5y+4& \\ 3x-8y=-2& \end{array}$
3(5y + 4) − 8y = −2
15y + 12 − 8y = −2
7y = −2 − 12
7y = −14
y = −2
x = 5y + 4 = 5 * (−2) + 4 = −10 + 4 = −6
Ответ: (−6; −2)