G(s) = I/V = 1 / (Ls+R+1/Cs)

G(jw) = 1 / [R+jwL+1/jwC] = 1 / [R+j(wL-1/wC)]

|G(jw)| = 1 / sqrt[ R² + (wL-1/wC)² ]

d/dw |G(jw)| = 0   =>.  w = 1/sqrt(LC)

w = 2 pi f   =>   f = 1/[ 2 pi sqrt(LC) ]