Test Tommorow, Urgent Discrete Math advice needed.

Having my first exam tomorrow, and while Im prepared, im finding that problems with modulus arithmetic are giving problems.

For example, prove that if ac =(congruent) bc (mod m), then a =(congruent) b (mod m/d), where d = gcd(m,c).

I was wondering if anyone could give me any tips for these types of problems?

This was a review question, and i WILL have a question like this on my exam tomorrow.

thanks!

btw i have the answer for the question, but just provided it as an example. I understand the theory and what it means, just being able to prove it one way or another is proving to be difficult.