∵tan2a=2tana/(1-tan²a)=2×0.5/(1-0.5²)=1/0.75=4/3
∴ tan(b-2a)=(tanb-tan2a)/(1+tanb×tan2a)
=(-0.4-4/3)/(1-0.4×4/3)
=(-2/5-4/3)/(1-2/5×4/3)
=(-26/15)/(7/15)
= -26/7