HOW DO I INPUT THE MIDPOINT RULE IN MATHEMATICA? I KNOW WHAT THE MIDPOINT RULE IS.

This is what I wrote but i'm not sure if it's correct. The problem is

17 where b-a =y-ity | = midpoint of [x, .. x ,] . and X, 17 where b-a =y-ity | = midpoint of [x, .. x ,] . and X