See here:
http://www.intmath.com/applications-integration/5-centroid-area.php
scroll down for a nice example, with steps for solution.
Find the centroid (¯ x, ¯ y) of the region bounded by:
y = 6x^2+7x, y = 0, x = 0,
and x = 7
2 answers
shut up steve they asked for the answer not for a link to a similar problem