$$ all strings over the alphabet $\{$ a$,$ b $\}$ that contain exactly two a$\text{'}$s

$$ all strings over the alphabet $\{$ a$,$ b $\}$ that do not end with ab

$$ all strings over the alphabet $\{$ a$,$ b $\}$ that contain an even number of b$$s$.$

$$ all strings which do not contain the substring ba