Most of intro econ is intersecting lines. Specifically,

$$MB = MC$$
[* Equilibrium is achieved when Marginal Benefit is equal to Marginal Cost*][1]

$$\dfrac{MU_x}{p_x}=\dfrac{MU_y}{p_y}.$$
[*Marginal Utility per unit cost should always be equal*][2]

Economics is about the logic of human behavior, how we make decisions in a world of scarcity. These equations describe constrained optimization. I'd also give prominence to consumer theory over producer. Most of undergrad producer theory can be understood with the same tools used in consumer theory.

[1]: http://faculty.winthrop.edu/stonebrakerr/book/market_efficiency.htm
[2]: http://dev.business.und.edu/Goenner/teaching/econ201/Resources/Book%20supplemental/07sg.pdf