Tell your pa you'll put more effort into it in the future.
If he's still pissed and doesn't believe you, come up with the idea that he could help you study. If he's a hypocrite, he'll chicken out because of all the scary math (like my parents did) and let it go, and if he's cool you might just get in some bonding time and a tutor. Either way, you'll at least sound like you're putting some thought into the matter, and he might go a little easier on you.
Goddamn, being a "grown-up" rocks. When you get bad grades in college you can just lie to your folks or tell them to piss off.