1. **State the problem:** Simplify the expression $a^2 + b^2 - (a - b^2)$. 2. **Recall the rule:** When subtracting a parenthesis, distribute the minus sign to each term inside: $-(a - b^2) = -a + b^2$. 3. **Apply the rule:** Rewrite the expression as $a^2 + b^2 - a + b^2$. 4. **Combine like terms:** Notice $b^2 + b^2 = 2b^2$, so the expression becomes $a^2 - a + 2b^2$. 5. **Final simplified expression:** $$a^2 - a + 2b^2$$ This is the simplest form of the given expression.