this is a proportion so
6/15=2/c
make the left side top number equal to the right side top number and you will find c
6/15=2/c
6/3=2
6/15 times (1/3)/(1/3)=2/5
2/5=2/c
c=5
the other way is multiply both sides by c/2
6c/30=1
multiply both sides by 30
6c=30
divide both sides by 6
c=5