Question 1114288
{{{y=a(x-h)^2+k}}}

points (2,1)​, (4,1)  mean that the vertex is on  the line x=3.

{{{y=a(x-3)^2+k}}}


Point (0,-3) makes for {{{-3=a(0-3)^2+k}}}
or  {{{9a+k=-3}}}.

Point (2,1) gives  {{{1=a(2-3)^2+k}}}
or  {{{a+k=1}}}.


Solve this system for a and k:
{{{system(9a+k=-3,a+k=1)}}}