Question 1138655
When it hits the ground, {{{ h(t) = 0 }}}
{{{ 0 = -16t^2 + 84t }}}
{{{ t*( -16t + 84 ) = 0 }}}
{{{ 16t = 84 }}}
{{{ t = 5.25 }}} sec
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check:
{{{ -16*5.25^2 + 84*5.25 = 0 }}}
{{{ -16*27.5625 + 441 = 0 }}}
{{{ -441 + 441 = 0 }}}
OK