[ Eq(Derivative(x1, t), (1*k7*k4*x6 + (-1)*k7*k2*x3)/k7),
  Eq(Derivative(x3, t), (2*k7*k3*x2*x6 + (-1)*k7*k2*x3 + (-1)*k7*k1*x4)/k7),
  Eq(Derivative(x4, t), (1*k7*k2*x3 + (-1)*k7*k1*x4)/k7),
  Eq(Derivative(x5, t), 0) ]