Given the following question:
Depending on the subject:
Equivalent expressions are when two or more expressions are equal to each other or are the same. By simplifying the expression to it's base you can see that. Equivalency is when you simplify one expression to it's base and it's not equal to the second expression making them both false when it comes to terms of them being equivalent. Common ways of checking equivalency which are logical reasoning (when you see that each equation has the same value in different models) and the truth table. (most common way of proving equivalency, breaking down componets of the two expressions to check if they're the same)