[ Eq(Derivative(x1, t), (1*k12*(k1 + k2*x4)*x2 + (-1)*k12*k4*x1)/k12),
  Eq(Derivative(x2, t), ((-1)*k12*(k1 + k2*x4)*x2 + 1*k12*k3*k13)/k12),
  Eq(Derivative(x3, t), 0) ]