Reduction of a language to a shorter equivalent
Think set theory.
We have for L1
$$ (A - {a}) \subset A^* $$
$$ A^*.(A-{a}).A^* \subset A^* $$
Then
$$ S1 = A^*.\{b\} $$
$$ S1 = A^*.(A)^*.A^*.\{b\} $$
$$ S2 =  (A^*.(A-{a})).A^*.\{b\} $$
$$ S1 - S2  = a^*b$$
Basically you removed everything from A* that does not have a, then you end up with a*.
Same argument goes for L2.
Then:
$$ L1.L2 = a*.b.b* = a^*.{b+b*} = a.b^+ $$
Note: This does not &quot;reduce&quot; the languages, but help you see how to arrive to the answer.
You may have to brush up on the operations of regular expressions, but the general logic should be a bit similar, if you realize that A* is a prefix closure and can be  written in many forms.