[tex]\displaystyle\bf\\\left \{ {{8x + 3y = 1} \atop {2x + 5y = - 21 \: \: | \times ( - 4) }} \right. \\ \displaystyle\bf\\ + \left \{ {{8x + 3y = 1} \atop { - 8x - 20y = 84 }} \right. \\ \\ 3y - 20y = 1 + 84 \\ - 17y = 85 \\ y = 85 \div ( - 17) \\ y = - 5 \\ \\ 2x + 5 \times ( - 5) = - 21 \\ 2x - 25 = - 21 \\ 2x = - 21 + 25 \\ 2x = 4 \\ x = 4 \div 2 \\ x = 2[/tex]

Ответ: ( 2 ; - 5 )