So, I have to find the solution to the congruencies
$x \equiv 4 \mod 315$
$x \equiv 9 \mod 715$
I know that I need to divide through by 5 to make the moduli coprime, and I know how to solve for $x$ from there. What I'm having trouble with is figuring out how to divide through by 5. I was given the following hint:
Hint: Let $y = x−4$, show that y must be divisible by $5$.
However, I'm not sure how I should show that $5 ~|~ x−4$.
Any help would be appreciated.