## cant understand certain part of a solution..

here is the problem and solution:
http://i26.tinypic.com/2vvm8vm.jpg
http://i31.tinypic.com/nqwq4n.jpg
i solved the 1st subquestion.
my problem starts with the second subquestion:
i understood how they got this expression
$$E=\frac{\epsilon_0}{\epsilon}E_0$$
but i dont know what they do next
what law principle are they doing in the next 3 lines??