x = - 4 - y;
Then, (-4 -y)y = -21;
Finally, we solve the question: y^2 + 4y - 21 = 0;
We have a = 1, b = 4, c = -21;
We calculate: b^2 - 4ac = 16 + 84 = 100;
We have y1 = ( - 4 + 10 ) / 2 = 3; y2 = (-4 - 10) / 2 = -7;
x1 = - 4 - 3 = - 7; x2 = -4 + 7 = 3;
