Let the smaller number be represented by x.

Then use the principle: 

If a letter represents one part of a given quantity, the other part of that
given quantity is represented by "That given quantity MINUS that part".

So the larger number is represented by 84-x.

That's 2x

That's the larger minus smaller or (84-x)-2x

That gives the equation 

(84-x)-2x = 9

Solve that for x, which will be the smaller number. Subtract from 84
to get the larger number.

You finish:

Edwin