Question 975673
<pre>
C = {x|x²=4}     <-- that's set-builder notation.

There are only two numbers whose squares are 4.  They are -2 and 2.

C = {x|x²=4} = {-2,2}   <-- that's roster notation.

If a set has n elements, it has 2<sup>n</sup> subsets.  So since C has 2 elements,
it has 2<sup>2</sup> or 4 subsets.  They are:

1. { } or <font size=5>&#8960;</font> <-- the empty set, which is a proper subset of every set except itself.
2. {-2)    <-- a one-element set, which is called a singleton.
3. {2}     <-- another singleton subset.
4. {-2,2}  <-- the whole set C, which is not a PROPER subset, but still a subset. 

Edwin</pre>