Solve Simple Equations
In an equation or an expression, x stands for a number that is not yet known (or specified). Sometimes we work with the expression involving x, other times the goal is to figure out what number x makes the equation true, other times we are given x and then asked to figure out what the equation will equal.
For solving simple equations, you can watch the video at https://www.khanacademy.org/math/cc-sixth-grade-math/cc-6th-expressions-and-variables/cc-6th-solving-equations/v/why-we-do-the-same--thing-to-both-sides--simple-equations.
This brings up a question of "why letters" in Algebra-https://www.khanacademy.org/math/algebra/introduction-to-algebra/overview_hist_alg/v/why-all-the-letters-in-algebra