use a geometric formula to evaluate the integral

integral from 0 to 3

$\displaystyle (2x+1)dx$

I integrated the problem, getting $\displaystyle x^2 + x$

Then I used the fundamental theorem of calculus, and I got 12-0 = 12..

Did I use a geometric formula or just calculus?? If I used Calculus, how do I find the answer via geometric formula??