Question 1062994
{{{a<0}}} for parabola to open downward.
{{{system(y=a(x-h)^2+k,a<0)}}}


You can pick any value for a that you want but it must be a negative value.


y-intercept 7
The point  (0,7).

{{{a(0-h)^2+k=7}}}
but your vertex  (h,k) is also still a variable.  



You specified only two things.  You could pick a specific factor a and vertex (h,k).   What if the vertex were also chosen to be the y-intercept?
What if {{{a=-1}}}?
{{{-1(x-0)^2+7=y}}}
{{{y=-x^2+7}}}------------just one example that fits.