for (a) let x = 0 and solve
for (b) find the line of symmetry of the curve
for a parabola
ax2+bx+c
the line of symmetry is
x=−b2a
when you get this value substitute it for the max height
(c) you need to find then the height is 0....
so let H(t) = 0 and solve for x... maybe the general quadratic formula is easiest...