Question 935019
You can't solve it because it's not an equation
You need an = sign somewhere
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What you probably have is:
{{{ h(t) = -16t^2 + vt }}}
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This plots the height, {{{ h }}} of an object
thrown upward with initial velocity {{{ v }}}
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Are you being asked for the maximum height
or for the time, {{{t }}} for the object 
to come back to ground?
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For it to come back to ground, just say:
{{{ h(t) = o }}} ( height = 0 )
{{{ -16t^2 + vt = 0 }}}
{{{ t*( -16t + v ) = 0 }}}
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There are 2 solutions:
{{{ t = 0 }}} ( the instant the object is thrown )
{{{ t = v/16 }}} ( the instant the object lands }
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So your answer is {{{ t = v/16 }}}
( units are ft, ft/sec, ft/sec2