Find the centroid of the region bounded by

y=3x^2+6x, y=0 and x=3.

So what I've done so far

0<x<3

0<y<3x^2+6x

those are the bounds

and i take the double integral using those bound to find m

but I don't know what function to use I originally used 3x^2+6x but thats not correct.