\(m g-T=m a....(1)\)
also Inertia solid cylinder is \(M R^{2} / 2\) Torque at centre of cylinder due to both string \(2 T \times R=I \alpha=M R^{2} \alpha / 2 \ldots(2)\)
also, \(a=\alpha R....(3)\)
From equations \((1),(2)\) and \((3)\)
we get Tension \(T=\frac{M m g}{M+4 m}\)