Question 834994
Use the quadratic equation,
{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
{{{x = (-(-16) +- sqrt( (-16)^2-4*3*21 ))/(2*3) }}} 
{{{x = (16 +- sqrt( 256-252 ))/(6) }}} 
{{{x = (16 +- sqrt( 4 ))/(6) }}}
{{{x = (16 +- 2)/(6) }}}
{{{x=18/6=3}}} and {{{x=14/6=7/3}}}
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{{{ graph( 300, 300, -2, 6, -4, 4, 3x^2 - 16x + 21) }}}