I did this exercise based on this post: http://www.mathhelpforum.com/math-he...ongruence.html

And I just want to know if I did this properly and if this is the right approach.

Adding gives

Comparing with the third equation:

Adding gives

So the solution is

While I'm here, can anyone tell me what the easiest way to find the inverse of a congruence is? I seem to struggle with that.