Write a non-ambiguous CFG generating the words over $\{a,b,c\}$ such that there is an occurrence of $c$ exactly at the middle, to its left there are only $a$’s (and there is at least one $a$), and to its right there are only $b$’s (and there is at least one $b$). Note that, since the only occurrence of $c$ must be exactly at the middle, the number of $a$’s must be equal to the number of $b$’s.