12x and 15x are unlike terms.
False
The terms which have the same literal coefficients raised to the same powers but may only differ in numerical coefficient are called similar or like terms.
For example:
(i) 3m and –7m are like terms.
(ii) z and 32z are like terms.
The terms which do not have the same literal coefficients raised to the same powers are called dissimilar or unlike terms.
For example:
(i) 9p and 9q are unlike terms.
(ii) x3 and y3 are unlike terms.