a
\(R =\left(\frac{ ma }{3}\right)+\left(\frac{ a }{2 m }\right)\)

\(\frac{ dR }{ dm }=\frac{ a }{3}-\frac{ a }{2 m ^{2}}=0\)

\(\frac{ a }{3}=\frac{ a }{2 m ^{2}}\)

\(m ^{2}=\frac{3}{2}\)

\(m =\sqrt{\frac{3}{2}}\)

\(x =3\)