Make each known side into a ratio like so...
75/6Â
(Because 75 is the shadow casted by the building, and 6 is the shadow casted by sarah)
and 50/xÂ
(50 is how tall the building is and sarah's height is what needs to be found)Â
Therefore,Â
75 Â Â 50
---- Â -----
 6    x
Do cross products;
50 x 6 = 75 x X
300 = 75xÂ
Divide by 75Â
x = 4ft