The parabola is opened down --- it means that the leading coefficient at the quadratic term is negative.


The zero level is at x= 0 and x= 100 feet.  Hence, the parabola has the form

    y(x) = a*x*(x-100)


with the unknown coefficient "a".  We know that y= 25 at x= 50 (half way from x= 0 to x= 100), so

    25 = a*50*(100-50) = a*50*(-50).


It gives

    a = -  = - .


Thus the final equation of the parabola is  

    y(x) = -0.01*x*(x-100).


You can transform it to any other equivalent form.