To subtract monomials, they have to be similar, that is to say, they must have the same literal part.
The difference of two or more monomials is another monomial whose literal part is the same and the coefficient is the difference between the coefficients.
axn − bxn = (a − b)xn
2x2y3z − 3x2y3z = 5x2y3z
The subtraction of two or more non-similar monomials is a polynomial.
2x2y3 + 3x2y3z