Construct a grammar over {$a,b$} whose language is {${a^mb^ia^n | i = m+n}$}
You must handle aXb and bYa sequences seperately. 
$\\S \rightarrow \  XY\ |\ \epsilon \ \\ X\rightarrow \ aXb\ | \ \epsilon \\ Y\rightarrow  \ bYa\ | \ \epsilon$