3lbs for $21 at that rate how would 9 lbs cost

Well we are going to use 3lbs for $21 as our rate. 
Lets set it up in a fraction. 
  
  3lbs 

  $21 

So we want to know how much 9lbs would cost.. 
Well lets set that up as a fraction as well. 
  9lbs 

    x 

We put x there because we dont know how much 9lbs will cost.

Now we know that in order to get 9lbs they just multipled 3lbs x 3 
And that gave us 9lbs!!!
3lbs x 3 = 9lbs 

Now when dealing with fractions we must always keep one rule in mind. 
What ever you do to the top you must do to the bottom. 
So we could multiply $21 by 3 and get our 9lbs cost!!
$21 x 3 = $63 

So this means we can finally say our answer! 
 Answer = 9lbs would cost $63 

Good Luck! :)