{ x3 = k7 - x4,
  x4 = k7*2*k3*k6/(k5*x1 - k3 + k4*k5*x1 + k6*k3 + ((k5*x1 - k3 + k4*k5*x1 + k6*k3)^2 - 4*(k5*x1 - k3)*k3*k6)^(1/2)) }