> 
> Prove that no matter what integer k you choose, the number $7k+3$ will
>  not be a perfect square.
> 
> 
> 

My thought when attempting this problem is to use a proof by contradiction, and suppose that there is some k where $7k = a^2 - 3$ but im unsure on how to prove it after this, maybe it has something to do with modulo which we just learned about briefly on Friday?