I am trying to understand the following statement. 

Let $A$ be a noetherian commutative ring and $\mathfrak a\subset A$ is an ideal. Suppose that the ring $A/\mathfrak a$ is flat over $A$, then $V(\mathfrak a)$ is open in $\operatorname{Spec}
(A)$. How to prove this (just using the standard definition of flatness)?