3a+5c=27
3a+3c=18
3a=18-3c
18-3c+5c=27
2c=27-18
2c=9
C=9/2
3a+3*9/2=18
3a=18-27/2
3a=(36-27)/2
3a=9/2
A=9/6
A=3/2