What is the range of a triangle of measures of 19 and 9.

It' not clear what you mean by "range of a triangle" .

One possibility is that you might mean:  "What is the possible range
for the length of the third side of the triangle ?" .  Here's how I do that one:

-- The 19 and the 9 are connected at one end.

-- When they're closed up, together, both pointing in the same direction,
their open ends are (19 - 9) = 10 apart.  That's the closest their open ends
can ever come to each other.  So the third side must be at least 10, or else
it couldn't join their open ends.

-- When the 19 and 9 are wide open, pointing in opposite directions, their
open ends are (19 + 9) = 28 apart.  That's the farthest their open ends can
ever be from each other.  So the third side can't be longer than 28, or else
the 19 and the 9 could never reach between its ends.

-- So the length of the third side of the triangle must be more than 10
and less than 28.

I sure hope that this is what your question means.
Otherwise, ignore everything I wrote.

It would be 10 because in range, you subtract the biggest and smallest numbers.