i have a difficulty in this problem. 

Let the differential system order $n$ : $$ \dfrac{d X}{dt}=f(X)$$
with $X=(x\_1,x\_2,...,x\_n)$.

Prove that if we denote $\tau=t\_2-t\_1,$ then for any solution $X$ for this system satisfying $X(t\_1)=X(t\_2)$, we have $$X(t+\tau)=X(t)$$.
can you help me in 
[System Differential Equation - problem](https://math.stackexchange.com/questions/499967/system-differential-equation-problem)
please