Like Terms

In $3x+2x = 15$, the unknown appears twice on the same side. Before you undo anything, collect the two terms into one. Three x's and two more x's make five x's, in the same way that three apples and two apples make five apples.

$$5x = 15$$

$$x = 3$$

After collecting, the equation has only one x-term. Divide both sides by 5 and you get $x = 3$.

Terms with the same letter to the same power are like terms. To combine like terms, add their coefficients: $3x+2x = 5x$, $7x-4x = 3x$, $x+x = 2x$. Unlike terms stay apart. $3x+2$ cannot be shortened, because an x-term and a plain number are different kinds of term.

Collect like terms whenever a side has more than one x-term. Solving is easier when each side is as short as it can be.