1. **State the problem:** We want to make $g$ the subject of the formula $$T = 2\pi \sqrt{\frac{l}{g}}.$$ 2. **Start with the given formula:** $$T = 2\pi \sqrt{\frac{l}{g}}.$$ 3. **Square both sides to eliminate the square root:** $$T^2 = (2\pi)^2 \frac{l}{g} = 4\pi^2 \frac{l}{g}.$$ 4. **Rewrite the equation:** $$T^2 = \frac{4\pi^2 l}{g}.$$ 5. **Multiply both sides by $g$ to isolate it in the numerator:** $$g T^2 = 4\pi^2 l.$$ 6. **Divide both sides by $T^2$ to solve for $g$:** $$g = \frac{4\pi^2 l}{T^2}.$$ 7. **Final formula with $g$ as the subject:** $$\boxed{g = \frac{4\pi^2 l}{T^2}}.$$ This means the acceleration due to gravity $g$ can be calculated if you know the length $l$ and the period $T$ of the pendulum.