system equation

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'