Like Terms

$2x+4x$ is two x's plus four x's, which makes six x's: $6x$. Terms with the same letter, raised to the same power, are called like terms, and they can be added into one term by adding their numbers.

Why does this work? It is the distributive property read backwards: $2x+4x$ is $\left(2+4\right)x$, which is $6x$. A letter on its own has the number 1 in front of it, so $7a+a$ is $8a$.

Unlike terms stay apart. $3x+5$ cannot become 8x, because 5 is not a number of x's; and $3x+2y$ stays as it is, since x and y can be different numbers. Three apples and five pears make eight pieces of fruit, but never eight apples.