[ Eq(Derivative(x1, t), (1*k4*k1*x1 + (-1)*k4*x1*x3)/k4),
  Eq(Derivative(x2, t), (1*k4*x1*x3 + (-1)*k4*k5*x2)/k4),
  Eq(Derivative(x3, t), (1*k4*k5*x2 + (-1)*k4*(k3*x1*x3 + k2*x3))/k4) ]