Question 1009294
{{{ m = -.04x^2 + 3.6x - 49 }}}
{{{ 30 = -.04x^2 + 3.6x - 49 }}}
{{{ -.04x^2 + 3.6x - 79 = 0 }}}
{{{ -4x^2 + 360x - 7900 = 0 }}}
{{{ -x^2 + 90x - 1975 = 0 }}}
Use quadratic formula
{{{ x = ( -90 +- sqrt( 90^2-4*(-1)*(-1975) ))/(2*(-1)) }}} 
{{{ x = ( -90 +- sqrt( 8100 - 7900 ))/(-2) }}} 
{{{ x = ( -90 +- sqrt( 200 ))/(-2) }}} 
{{{ x = ( -90 + 14.142 )/(-2) }}} 
{{{ x = 37.93 }}} 
38 mi/hr
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and, also:
{{{ x = ( -90 - 14.142 )/(-2) }}} 
{{{ x = ( - 104.142 ) / ( -2 ) }}}
{{{ x = 52.07 }}}
52 mi/hr
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check:
{{{ 30 = -.04x^2 + 3.6x - 49 }}}
{{{ 30 = -.04*38^2 + 3.6*38 - 49 }}}
{{{ 30 = -.04*1444 + 136.8 - 49 }}}
{{{ 30 = -57.76 + 136.8 - 49 }}}
{{{ 30 = 30.04 }}}
close enough
You can check the other solution