I Don't Understand This Joke

[SIZE="2"]I saw this shirt on someone passing by and I solved it... but I still don't get it. Maybe I made a mistake. But I found the antiderivative of the integral which should be x^2. Then I figured (10)^2 = 100 and (13)^2 = 169. Therefore 100-169= -69+C. When I asked the guy what the shirt meant, he said it was some sort of sexual joke. But the only possible way that could be a sexual joke is if you take it to mean "position 69" which is a by-word sexual pose. But this still doesn't explain the negative preceding 69 and the constant after it.

Did I do the integral wrong or subtract the values out of order? I still don't understand how this is a viable sexual joke. [/SIZE]