Answer
See below.
Work Step by Step
The margin of error of a sample size $n$ is a $95\%$ probability that a sample lies within the margin of error which is $\pm\frac{1}{\sqrt n}\cdot 100\%$
a) Hence here the margin of error: $\pm\frac{1}{\sqrt {4000}}\cdot 100\%\approx\pm1.6\%$
b) Thus we can be $95\% $confident that between $51.1-1.6=49.5\%$ and $51.1+1.6=52.7\%$ of the population tuned into the episode.
