Solve.
-A projectile is thrown upward so that its distance above the ground after $t$ seconds is $h ( t ) = - 16 t ^ { 2 } + 420 t$. After how many seconds does it reach its maximum height?
A) $7 \mathrm { sec }$
B) $28 \mathrm { sec }$
C) $21 \mathrm { sec }$
D) $13 \mathrm { sec }$

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The Answer of Solve.
-A projectile is thrown upward so that its distance above the ground after $t$ seconds is $h ( t ) = - 16 t ^ { 2 } + 420 t$. After how many seconds does it reach its maximum height?
A) $7 \mathrm { sec }$
B) $28 \mathrm { sec }$
C) $21 \mathrm { sec }$
D) $13 \mathrm { sec }$

Solve -A Projectile Is Thrown Upward So That Its Distance ttt

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-A Projectile Is Thrown Upward So That Its Distance $t$