Final answer:
To simplify the expression 21b^4 - 3b by combining like terms, we add or subtract the coefficients of the terms with the same exponent.
Step-by-step explanation:
To simplify the expression 21b^4 - 3b by combining like terms, we add or subtract the coefficients of the terms with the same exponent. In this case, the terms 21b^4 and -3b have the same variable (b) and the same exponent (4), so we can combine them.
The simplified expression is 21b^4 - 3b.