Question

Using the equation T=2π√L/g solve for L

Answer 1

You said T = 2 · π · √(L / g)

(I think that's the formula for the full-swing period of a simple pendulum.)

Divide each side by 2π : T/2π = √(L / g)

Square each side: (T/2π)² = L / g

Multiply each side by ' g ' : L = g · (T/2π)²