a) Since we are interested in the number of TVs that can be sold at $400, we need to use the Demand model equation and set p=400; thus,

The answer to part a) is 20 TVs per week.
b) Set N=N, then

Therefore, using p=300 and solving for N,

The answer to part b) is 720 TVs per week.
c) In part b), we found that when supply and demand are equal, p=300. Thus, the answer to part c) is $300