How would I find the "how high will it go" answer to this equation?

Question

An archer shoots an arrow straight up at an initial velocity or 4096 ft/sec. How fast is this mph? How high will it go?

Answer 1

h = -16 t^2 +4096 t
h´= -32t+4096 = 0 so t for max height = 128 s (not real)
and max height =262,144 feet (nonsense)
it would be 4096/5280 *3600 mph=2792.73 mph
check this problem