Give a regular expression for the following sets

Explain the expression by writing what each part does.

(a) L={w | w {a,b} and w begins and ends with aa or bb}

(b) L={w | w {a,b} and the number of as in w is divisible by three}

Construct a DFA for the following sets. The sets are the same as the previous question. Explain the DFA by writing what each part does or what each state corresponds to.