[tex]\displaystyle\bf\\\left \{ {{25 {x}^{2} - 4 {y}^{2} = 21} \atop {5x - 2y = 7 }} \right. \\ \displaystyle\bf\\\left \{ {{25(0.4y + 1.4) {}^{2} - 4 {y}^{2} = 21 } \atop {x = 0.4y + 1.4 }} \right. \\ \\ 25(0.16y {}^{2} + 1.12y + 1.96) - 4 {y}^{2} = 21 \\ 4 {y}^{2} + 28y + 49 - 4 {y}^{2} = 21 \\ 28y = 21 - 49 \\ 28y = - 28 \\ y = - 28 \div 28 \\ y = - 1 \\ \\ x = 0.4 \times ( - 1) + 1.4 = - 0.4 + 1.4 = 1[/tex]

Ответ: ( 1 ; - 1 )