1. **State the problem:** Simplify the expression $a^2 + a^2 - 36$ when $a = 3$. 2. **Write the formula and rules:** The expression involves squaring a variable and combining like terms. Recall that $a^2$ means $a$ multiplied by itself. 3. **Substitute the value:** Replace $a$ with 3 in the expression: $$3^2 + 3^2 - 36$$ 4. **Calculate each term:** $$3^2 = 9$$ So the expression becomes: $$9 + 9 - 36$$ 5. **Combine like terms:** $$9 + 9 = 18$$ So: $$18 - 36$$ 6. **Perform the subtraction:** $$18 - 36 = -18$$ **Final answer:** $$-18$$