3x - 4y + 9

Take out the x and put (2) in its place
Take out the y and put (5) in its place:
Then you have:

        3(2) - 4(5) + 9

Now we must do multiplications before additions
and subtractions, so take out the 3(2) and put
6 (which is 3 times 2) in its place.

Also take out the 4(5) and put 20 (which is 4
times 5) in its place. Then we have:

           6 - 20 + 9

Now there is only a subtraction and an addition
left to do, so we must do them in the order of
left to right.  We come to 6 - 20 first, so we 
do that and get -14.  Then we replace 6 - 20 by
-14.  Then we have

             -14 + 9

Now we do that addition and end up with -5.

So the answer is -5.

Edwin