{ x3 = 2*k4*x1*k7/(k6 - k4*x1 + k5*k6 + k7*k4*x1 + ((k6 - k4*x1 + k5*k6 + k7*k4*x1)^2 - 4*(k6 - k4*x1)*k4*x1*k7)^(1/2)),
  x4 = k8 - x3 }