oggers... To clarify further:
You seem to understand that Power required = drag x airspeed.
Drag is the same as Thrust required for level flight; it's basically the force that needs to be offset with thrust so the airplane can fly.... therefore,
Power required = thrust required x airspeed
And Power available is,
Power available = thrust available x airspeed
This picture shows it:
http://i.imgur.com/R43ho.png
What might be confusing is the Thrust Horsepower term. It is still
power and is talking about the power generated by the thrust that propels the aircraft forward. If the picture was showing the BHP or SHP as well, it would be
virtually a straight line near the top of the graph.
Here is a picture I took from another textbook of mine that shows the THP available and required curves. It's not 100% clear but I think it shows enough:
http://i.imgur.com/3e4Ru.jpg?1