T=2pi√I/g. find I

T = 2 π √ ( I / g )

Square both sides of the equation

T² = ( 2 π )² ∙ [ √ ( I / g ) ]²

T² = 4 π ² ∙ I / g

Multiply both sides by g

T² ∙ g = 4 π ² ∙ I

Divide both sides by 4 π ²

T² ∙ g / 4 π ² = I

l = T² ∙ g / 4 π ²

l = ( T² / 4 π ² ) ∙ g

l = ( T / 2 π )² ∙ g