Problem $3$

The acceteration due to gravity at an alitude $y$ above the earth's surface is:

$a=\frac{-32\mathrm{.}2}{[1+(\frac{y}{20\mathrm{.}9}{10}^6){]}^2}$

Where, the units of a are in $\frac{ft}{s^2}$ and y are in ft $.$Find the height reached by the projectile if it is fired

vertically upward with an initial velocity of $6000\frac{ft}{s}.$