Question 1030926
{{{ y = -6x^2 + 36x }}}
You want 2 solutions for {{{ y = 48 }}}
{{{ 48 = -6x^2 + 36x }}}
{{{ -6x^2 + 36x = 48 }}} ( same equation )
Divide both sides by {{{ -6 }}}
{{{ x^2 - 6x = -8 }}}
Complete the square
{{{ x^2 - 6x + (6/2)^2 = -8 + (6/2)^2 }}}
{{{ x^2 - 6x + 9 = -8 + 9 }}}
{{{ ( x - 3 )^2 = 1 }}}
Take the square root of both sides
{{{ x - 3 = 1 }}}
{{{ x = 4 }}}
There is a 2nd answer which you get by 
taking the negative square root of 1 also:
{{{ ( x - 3 )^2 = 1 }}}
{{{ x - 3 = -1 }}}
{{{ x  = 2 }}}
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The ball is at 48 ft 2 sec after it is hit and
also at 48 ft 4 sec after it is hit
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Here's the plot:
{{{ graph( 400, 400, -2, 10, -6, 60, -6x^2 + 36x ) }}}