[ Eq(Derivative(x1, t), (-1)*k7*k3*x1*x3/x6/k7),
  Eq(Derivative(x2, t), (1*k7*k3*x1*x3/x6 + (-1)*k7*k4*x2)/k7),
  Eq(Derivative(x3, t), (1*k7*k4*x2 + (-1)*k7*k5*x3 + (-1)*k7*k6*x3)/k7),
  Eq(Derivative(x4, t), 1*k7*k5*x3/k7),
  Eq(Derivative(x5, t), 1*k7*k6*x3/k7),
  Eq(Derivative(x6, t), 0/k7) ]