1. **State the problem:** Simplify the expression by collecting like terms: $$a^2 + a^2 + a^2 + a$$. 2. **Identify like terms:** The terms $$a^2$$ are like terms because they have the same variable and exponent. 3. **Combine like terms:** There are three $$a^2$$ terms, so add their coefficients: $$a^2 + a^2 + a^2 = 3a^2$$. 4. **Rewrite the expression:** Now the expression becomes: $$3a^2 + a$$. 5. **Final simplified expression:** The expression simplified by collecting like terms is: $$\boxed{3a^2 + a}$$.