Point B: Distance of B from lens is 12 cm i.e., At 2 F so image of B is formed at distance 12 cm on the other side.
image of A: `u=-(11-1)=-10cm,f=6cm,v=?`
`(1)/(v)-(1)/(-10)=(1)/(6)implies(1)/(v)=(1)/(6)-(1)/(10)=(10-6)/(60)=(1)/(15)`
Image length `A^(`)B^(`)=15-12=3cm`