Question

(a) has 2 subsets how do I find the subsets?

Answer 1

i think your missing parts to the question here. Subset to me means that this is speaking about set theory. And there might be something else to the problem here.

If {a} is the only element in the set, then we have {a} and {null} as two subsets of {a}.

But without further information, thats just a guess :)

Answer 2

a subset of a set consists of all the possible permutations of the elements in that set including the set itself and the null set.

Answer 3

This is also called the power set of a set. A way to know how many subsets are in a given set, you can use the proven theorem:
\[|\rho({a})|=2^{\left| a \right|}\]
Where the bars represent the cardinality, i.e. the number of elements in the set.
For example, {a} has one element, so the number of subsets it has is \[2^{|a|}=2^{1}=2\]
Just a thought in case you need to do it for more than one element.