3c + 5t = 39....multiply by -3
9c + 7t = 63
----------------
-9c - 15t = - 117 (result of multiplying by -3)
9c + 7t = 63
---------------add
- 8t = - 54
t = -54/-8
t = 6.75 <=== tables are 6.75
3c + 5t = 39
3c + 5(6.75) = 39
3c + 33.75 = 39
3c = 39 - 33.75
3c = 5.25
c = 5.25/3
c = 1.75 <=== 1.75 per chair