To solve this problem, we need to determine the accelerations of blocks A and B immediately after the string is cut.$\\$Initially, the system is in equilibrium, so the spring force balances the gravitational force on the entire system.$\\$When the string is cut, block B is no longer supported by block A, and both blocks will experience free fall.$\\$Let's analyze each block separately:$\\$1. Block A (mass $3m$):$\\$• After the string is cut, block A is only acted upon by gravity.