{ x3 = k9 - x4,
  x4 = k9*2*k5*k8/(k7*x1 - k5 + k6*k7*x1 + k8*k5 + ((k7*x1 - k5 + k6*k7*x1 + k8*k5)^2 - 4*(k7*x1 - k5)*k5*k8)^(1/2)) }