x + y = 6  ==> y = 6 - x
 x^2 + y^2 = 68

 x^2 + (6 - x)^2 = 68==> x^2 + 36 -12x + x^2 - 68 = 0
 2x^2 - 12x - 32 = 0 (:2)
  x^2 - 6x - 16 = 0

delta = (-6)^2 -4.1.(-16)==> 36+64==> 100

x = 6+/V100 ==> x = 6+/-10
            2.1                      2

x1= 6+10 ==> x1 = 8
          2
x2= 6-10 ==> x2 = - 2
          2

| 8 | - | - 2 | = 8 - 2 ==> 6