(19/4a²+5/2ab-28/3b²)-(-5/4a²-23/6ab+3/4b²)
=19/4a²+5/2ab-28/3b²+5/4a²+23/6ab-3/4b²
=(19/4a²+5/4a²)+(5/2ab+23/6ab)+(-28/3b²-3/4b²)
=6a²+19/3ab-121/12b²