$15x$ is a variable.

There are three terms in the expression $xyz$.

Like terms can have different variables.

$5xy\text{ and }5pq$ are like terms.

The coefficient of $2x^{3}y$ in the term $12x^3{y}^{2}z\text{ is }6yz$.

At $x=1$, the value of algebraic expression $12x^3-10x^2+5x-9\text{ is }2$.

The value of an algebraic expression does not change with the change in the value of the variable.