2x+y=7 y=x^2-1 система

2x+y=7
y=x^2-1

подстановка
2x+x^2 - 1 =7
x^2 + 2x -8 = 0
D = d^2 - 4ac = 2^2 - 4 * 1 *(-8) = 4+32 = 36 = 6^2
x1 = (-b + (D)^1/2 )/ 2a = (-2 + 6) / 2 = 2
x2 = (-b - (D)^1/2 )/ 2a = (-2 - 6) / 2 = -4

y1 = 2^2- 1 = 4-1= 3;
y2 = -4^2 -1 = 16-1=15;

проверка
(x = 2, y = 3):
2*2+3=7   => 7=7
3 = 2^2 - 1   => 3=3

(x = -4, y = 15)
2*(-4)+15=7   => -8+15 = 7 =>  7=7
15 = (-4)^2 -1   => 15 = 16-1   => 15=15