Rotational Motion Problem - Unsure of my answer.

You are the technical consultant for an action-adventurefilm in which a stunt calls for the hero to drop off a 20.0-m-tallbuilding and land on the ground safely at a final vertical speedof 4.00 m/s. At the edge of the building’s roof, there is a 100.-kgdrum that is wound with a sufficiently long rope (of negligible mass) has a radius of 0.500 m,and is free to rotate about itscylindrical axis with a momentof inertia *I*0. The script calls for the 50.0-kg stuntman to tiethe rope around his waist andwalk off the roof.

a. Determine an expressionfor the stuntman’s linearacceleration in terms of hismass *m*, the drum’s radius *r,*and moment of inertia *I*.

b. Determine the required value of the stuntman’s accelerationif he is to land safely at a speed of 4.00 m/s, and use this valueto calculate the moment of inertia of the drum about its axis.

Now for a I obtained the following relationship: a= (mgr^2) /(I - mr^2) . Now the issue with this expression is that its also in terms of g. But I'm not sure how else to change it.

And for part b, since the rope is negligible in mass is it fair to assume that the drum is a disk -> so I=1/2mr^2?