Let H be the height of the taller building and h be the height of the other building.
(H-h)/12=tan20, h/12=tan35, so H-h=12tan20, h=12tan35=8.40m approx.
H=12(tan20+tan35)=12.77m approx.
Height of tall building is BC=12.77m, height of smaller building is 8.40m (approx figures).