So I get:
ΣF = mg - FT = ma
FT = m(g - a)
= .3kg (9.8 - 0.12m/s^2)
= 2.9 N
Then for part c (mass of cylinder), I tried using α = a/r, τ = FTr, I = 1/2MR^2, and τR = Iα to solve for mass, but I'm still left with r from α = a/r
FTr x R = I α
FT R^2 = 1/2MR^2 α
FT = 1/2M a/r