\( {x^2} = \frac{{3{a^2}}}{4} \) ==> \( x = \frac{{\sqrt 3 \,a}}{2} \)
The moment of inertia of system about \(AB\) side of triangle
\(I = {I_A} + {I_B} + {I_C}\)
\( = 0 + 0 + m{x^2}\)
\( = m{\left( {\frac{{a\sqrt 3 }}{2}} \right)^2}\)\( = \frac{3} {4}m{a^2}\)