x + y = 2 => x =2 - y
xy=-15
(2-y)y=-15 => 2y-y^2 =-15 => -y^2 +2y + 15 =0 => y^2 - 2y -15 =0
a = 1; b = -2; c = -15
delta = b^2-4ac = (-2)^2 -4 (1) (-15) = 4 + 60 = 64
y1 = (-b+√delta)/2a = (2 + 8)/2 =5
y2 = (-b-√delta)/2a =( 2-8)/2 = -3
x1= 2 - y1 = 2 - 5 = -3
x2 = 2 - y2 = 2 - (-3) = 5
(x ; y) = { (-3; 5); (5; -3) }