syntax error at medias_2.vhd(31) near text "in"; expecting "(",

I dont know, as line 31 in the code you posted is a blank line.

This is a pretty simple error to find and fix yourself. Just get a decent text editor - they all do bracket matching to ensure each ( has a corresponding )

And again - I cant see your real code, so no idea whether removing std_logic_arith is going to help (but you shouldnt use it anyway)