Use the Language of Algebra: example 1.22

$y^3$ and $4y^3$ are like terms because both have $y^3;$ the variable and the exponent match.

$7x^2$ and $5x^2$ are like terms because both have $x^2;$ the variable and the exponent match.

14 and 23 are like terms because both are constants.

There is no other term like 9x.