Question 884161
Your long part tells the distance between main vertices.  This is 4, so {{{a=4/2=2}}}.


Your short part tells about b.  {{{b=1}}}.


The important relationship to know for an ellipse is {{{b^2+c^2=a^2}}}.  Understand that c is the distance from the center of the ellipse to either focus.

{{{c^2=a^2-b^2}}}
{{{c^2=2^2-1^2}}}
{{{c^2=4-1}}}
{{{c=sqrt(3)}}}


To find either focus, you want to know the center.
Your given points allow you to observe or calculate center is at (-1,0).


Foci:  ( {{{sqrt(3)-1}}},0 ) and  (  {{{-sqrt(3)-1}}}, 0 )