\( 21 a ^ { 2 } x ^ { 2 } y \)

This expression represents a term from algebra that involves variables and coefficients. Here, \(21\) is the coefficient, which tells us how many times we multiply the other variables \(a^2\), \(x^2\), and \(y\). In algebra, variables can represent anything from quantities in a mathematical problem, so understanding this expression helps in simplifying equations and solving problems where these variables might stand for specific values. When dealing with such algebraic expressions, one common mistake is to forget the rules of exponents when combining like terms. For example, if you had \(a^2 \cdot a^3\), you would add the exponents to get \(a^5\). It's also crucial to keep track of the coefficients, as they can significantly affect the outcome of your calculations when you're adding or multiplying similar terms.