What is the most efficient way to effect a voltage drop in a dc cct?
You can't really use a resistor because voltage drop depends on current draw. If you choose a resistance value to drop 5V at 2.1A then if at any time the TV draws less than 2.1A the output voltage will rise above 19V.

So you need a circuit that regulates the output voltage so it remains stable when load current varies. The two main classes of regulator are linear and switching.

A linear regulator acts like a variable resistor which is continuously adjusted to keep the output voltage constant. It suffers the same power dissipation problems as a resistor, but has the advantage of a relatively simple circuit. 

Switching regulators pulse current through an inductor to 'transform' the voltage down. They can be over 90% efficient because the inductor stores and releases the pulse energy rather than wasting it as heat. The disadvantage is a more complex circuit with critical component specifications and circuit layout. 

Using a linear regulator to drop 24V down to 19V at 2.1A would waste (24-19)*2.1 = 10.5W. You would need a fairly large heat sink to remove this heat. However the circuit efficiency would be 79% which is not bad. A 90% efficient switching regulator would only dissipate 4.4W. 

The LM2596 is a switching regulator IC that only needs a few components to complete the circuit. Here's an example from the datasheet:-



However this apparent simplicity is a bit deceptive because for best performance the inductor characteristics have to be carefully matched to the input and output voltages and current. 

You could try building a switching regulator using the LM2596 with the datasheet to guide you, or just buy a ready-made module that uses a similar chip. These are cheap and readily available from eBay and electronic hobbyist suppliers (eg. IC Station).