means 

"the exponent to which we must raise 'b' to get 'a'"

That's why 

It's 2 because 2 is the exponent to which we must raise 5 to get 25.

Your problem 



says in words:

8 raised to the exponent to which 8 must be raised to get 19.

When you do whatever it is necessary to do in order to accomplish 
a task, you accomplish the task, right?

So when you do to 8 what you must do to 8 to get 19, then you
get 19.

Answer: 19.

Try to think through those words.  It's tricky wording, but it's 
a case of this:

"When you do what you have to do in order to get X, then
you get X."

Edwin