A regular expression for a given formal language
Try and build it step by step:


a language with an odd number of $a$'s: $a(aa)^*$;
between which there is an even number of $b$'s: $b^*a((bb)^*a(bb)^*a)^*b^*$;
and an arbitrary number of $x$'s: $(b+x)^*a(x^*(bx^*bx^*)^*ax^*(bx^*bx^*)^*a)^*(b+x)^*$.