6.EE.A.2
Write, read, and evaluate expressions in which letters stand for numbers.
Students will learn how to identify terms, constant terms, factors, and coefficients in algebraic expressions. Here's a good demonstration. Students can practice this skill here. Conclude by giving your students this challenge.
Notes: I don't think the Khan Academy exercise Writing expressions with variables should be included. I can't imagine a situation where someone would describe these expressions in English instead of math. It's just as absurd as describing the Pythagorean theorem in English as a squared plus b squared equals c squared. Who would do such a thing?