(a+2)/3=b/4
2(a+2)=6b/4
2a=6b/4-4
b/4=(c+5)/6
18b/4=3c+15
3c=18b/4-15
2a-b+3c=6b/4-4-b+18b/4-15=5b-19=21
5b=40
b=8
c=7
a=4
a:b:c=4:8:7