在这个方程中，x = 6。

In this ___equation, x = 6.

我想用一个非常简单的经济学方程来解决这个问题：需求 = 供应。

I would like to address the issue using a very simple ___equation of economics: Demand = supply.

薛定谔方程是量子力学的基础。

The Schrödinger ___equation is fundamental in quantum mechanics.