Well lets start with an equation.
Let's say 3b=6a, in order to solve this you would first have to choose a variable and isolate it
b=(6a)/3, With the new isolated variable you can now plug in.
3((6a)/3)=6a, By working this out you should have isolated the variable "a"
a=1, plug into the original equation and solve for the remaining variable
3b=6(1) -> 3b=6 -> b=6/3 -> b=2
hopes this helps