It's in the Love Spanked episode of Rocko's Modern Life where Rocko is in a car with a date, and she says "How bout we trade math equations baby!".

I get its an innuendo, but what exactly does it mean?

Also, I don't ger this joke either from an other episode:

"At one point when Heffer exercises, a woman comes up and asks if he needs any help, to which he replies. "Yeah" then holds up a plate and a pair of chopsticks, and says, "These plastic things can't cut my burrito.""