1. **State the problem:** Evaluate the expression $4t^2 - (2t)^2$ when $t=3$. 2. **Write the formula and explain:** The expression is $4t^2 - (2t)^2$. We will substitute $t=3$ and simplify step-by-step. 3. **Substitute $t=3$:** $$4(3)^2 - (2 \times 3)^2$$ 4. **Calculate powers and multiplication:** $$4 \times 9 - (6)^2$$ 5. **Simplify powers:** $$36 - 36$$ 6. **Subtract:** $$36 - 36 = 0$$ 7. **Final answer:** The value of the expression when $t=3$ is $0$.