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

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