Q:  What number do we want to make less?
A:  The 19.
Q:  How much do we want to make it less?
A:  By 17.
Q:  How do we make something less?
A:  By subtracting from it.
Q:  So what do we subtract from what?
A:  We subtract the 17 from the 19.
Q:  So which one do we put down first?
A:  The one that we want to make less, the 19.

So we put down the 

19   

and then we want to make it less, so we subtract 17 from it,
and we have

19 - 17

The answer is 2.

Edwin