1. **State the problem:** Simplify the expression $28y^2 - 21y$. 2. **Identify the common factors:** Both terms have a common factor of 7 and a factor of $y$. 3. **Factor out the greatest common factor (GCF):** $$28y^2 - 21y = 7y(\cancel{4}y - \cancel{3})$$ 4. **Simplify inside the parentheses:** $$7y(4y - 3)$$ 5. **Final answer:** The expression factored is $7y(4y - 3)$. This means we took out the largest common factor from both terms to simplify the expression.