Just consider the hillock to be a straight verical line. let its height be h and the distance of the observer from the foot of the line be x. therefore angle of elevation from this point is 45 degrees. hence tan(45) = h/x => h/x = 1 => h=x.
Now when we move closer to the foot by 100 m the angle of elevation increases to 60 degrees. therefore
tan(60) = h/x-100
=> √3 = h/h-100
=> h*(√3 - 1) = 100√3
=> h = 50*(3+√3) = 50*4.732 = 236.6 m