Answer :

c=18 3/7

c/3= 6 1/7 (change to improper) (7*6 +1)
c/3 = 43/7
c/3 *3 = 43/7 * 3 (*3 to both side to isolate c)
c= 47/3 * 3
c= 129/7 (change to mix)
c= 18 3/7