I am using == to denote congruency:

a== b (modm) meaning a is congruent to b (modm)

(or m divides a-b)

Suppose A is the set of integers

Let a,b,m be elements of A such that m>=2.

Prove "If a == b(modm) then gcd(a,m)=gcd(b,m)"

Everytime I try to do this, I get stuck.

Thank you for your help.