Two blocks A and B of masses $3m$ and $m$ respectively are connected by a massless string and suspended by a massless spring. The magnitudes of acceleration of A and B immediately after the string is cut, are respectively:

Before the string is cut:The system is in equilibrium.Tension in string $T=mg$.Spring force $F_s=(3m+m)g=4mg$.Immediately after the string is cut ($T\to 0$):For block B: Only gravity acts. $mg=m{a}_B⟹a_B=g$ (downwards).For block A: Spring force $4mg$ acts up, gravity $3mg$ acts down.
$$F_{\text{net}}=4mg-3mg=mg$$
$$a_A=\frac{mg}{3m}=\frac{g}{3}\text{ (upwards)}$$
Accelerations are $\frac{g}{3}$ and $g$.