Show that $\mathcal{M} \models \sigma(\vec{a})$ if and only if $\mathcal{M} \models \sigma(\vec{b})$.

Question

So I have this question
Let $\sigma$ be a sentence in a language $L, \mathcal{M}$ an $L$ -structure, and $\vec{a}, \vec{b}$ infinite tuples from $M .$ Show that $\mathcal{M} \models \sigma(\vec{a})$ if and only if $\mathcal{M} \models \sigma(\vec{b})$.

I don't know how to approach this question. Can somebody please help me?