Question 1088921
This is a parabola with a peak.
{{{ x[max] = -b/(2a) }}}
{{{ x[max] = -5/2 }}}
and
{{{ y[max] = -x^2 - 5x + 6 }}}
{{{ y[max] = -(-5/2)^2 - 5*(-5/2) + 6 }}}
{{{ y[max] = -25/4 + 25/2 + 6 }}}
{{{ y[max] = 25/4 + 24/4 }}}
{{{ y[max] = 49/4 }}}
The range is 
49/4 > = f(x) > minus infinity
Here's the plot:
{{{ graph( 400, 400, -10, 10, -10, 16, -x^2 - 5x + 6 ) }}}
check the math & get another opinion if needed