Question 201859
Notice that the quadratic {{{f(x)=-x^2-18}}} is in the form of {{{f(x)=Ax^2+Bx+C}}} where {{{A=-1}}}, {{{B=0}}}, and {{{C=-18}}}




Recall that


if A > 0, then the parabola opens upward


if A < 0, then the parabola opens downward



Since {{{A=-1}}}, which is less than 0, this means that the parabola opens downward.



Here's a graph to prove it:



{{{ drawing(500, 500, -20, 20, -25, 15,
 graph( 500, 500, -20, 20, -25, 15,-x^2-18)

)}}}



Graph of {{{f(x)=-x^2-18}}}