What are a, b, and c?
Hmm... To me, this looks like maybe homework? Instead of giving you the answer, here are some hints:
Add the last two equations together, what do you get?
From the second equation b = a+7 sub this into the third equation ...then solve for 'a'
use this value of 'a' in equation 2 or three to find 'b'
then use the values of 'a' and 'b' in equation 1 to find 'c'