Let the height of the desk be h m. Time of fall of the eraser from the top of the desk being t=0,77s we can write by considering the motion under gravity

h=0xt+1/2gt^{2 }

h=1/2x9.80x0.77^{2}=1.45 m

If the height of the school building be H m and angle of elevation of sun be A then

h/H=H/S=tanA , where s and S are length of shadows of desks and hall.

given s =2.92 m. S =8h=8*1.45=7.6 m

now h/H=s/S=2.92/7.6

So H = 7.6/2.92*h = 7.6/2.92*1.45 m=3.77 m