7. Find regular expressions for the following languages over alphabet {a, b}

(a) L1 = {w | | w | mod 3 0}

(b) L2 = {w | n_a(w)mod3 = 0} where na(w) is the number of as in the string w.

(c) L3 = {aⁿ b^m | n 2, m 1, nm 3}

(d)L4 is the complement of the language L = {a²ⁿ b^2m+1 | n, m 0}