well you would have to multiply 3 by what ever (y) is so if (y) was 5 then you could put the equation many ways but just some ways are
1) 3(y)=15
3(5)=15
15=15
2)3*y=15
3*5=15
15=15
just two ways and remember what you do to one side you have to do to the other but in this case you leave the right side alone