Hi,
Note: the vertex form of a Parabola opening up(a>0) or down(a<0),
where(h,k) is the vertex and x = h is the Line of Symmetry
h = - 16 t^2 + 12x + 10
h = -16(x - 3/8) + 16(9/64) + 10
h = -16(x - 3/8) + 9/4 + 40/4 \ parabola opening Downward a = -16 < 0
h = -16(x - 3/8) + 49/4 Vmax(3/8, 12.25)
12.25 ft the maximum height that the ball will reach