We can solve this by setting up a ratio comparing the height of the building to the height of the man and shadow of the building to the shadow of the man.
hb= height of the building
sb= shadow of the building
hm= height of the man
sm= shadow of the man
hb/sb = hm/sm
sb=x
45ft/x=6ft/8ft
x=(45x8)ft/6ft
x=60ft