Two Letters

An expression can hold more than one letter. In $3x+2y+x-5y$, the x terms are like each other and the y terms are like each other, but an x term and a y term are unlike. Collect each letter on its own.

The x terms are $3x+x$, which is $4x$. The y terms are $2y-5y$, which is $-3y$. So the expression simplifies to $4x-3y$, and that is as far as it goes.

Why stop there? x and y can be different numbers, so 4x - 3y has no single number of anything to combine into. It is like adding 4 metres to 3 kilograms: two quantities, written side by side.