Let the point on the locus be #(x,y)#, sum of whose distances from #(3,1)# and #(-1,1)# is #6#, hence
#sqrt((x-3)^2+(y-1)^2)+sqrt((x+1)^2+(y-1)^2)=6# or
#sqrt((x-3)^2+(y-1)^2)=6-sqrt((x+1)^2+(y-1)^2)#
Squaring each side, we get
#(x-3)^2+(y-1)^2=36+(x+1)^2+(y-1)^2-12sqrt((x+1)^2+(y-1)^2)# or
#x^2-6x+9+y^2-2y+1=36+x^2+2x+1+y^2-2y+1-12sqrt((x+1)^2+(y-1)^2)# or
#-6x+9=36+2x+1-12sqrt((x+1)^2+(y-1)^2)# or
#12sqrt((x+1)^2+(y-1)^2)=36+2x+1+6x-9=8x+28# or
#3sqrt((x+1)^2+(y-1)^2)=2x+7# and squaring again
#9((x+1)^2+(y-1)^2)=4x^2+28x+49# or
#9(x^2+2x+1+y^2-2y+1)=4x^2+28x+49# or
#5x^2+9y^2-10x-18y-31=0#
As the coefficient of #x^2# and #y^2# are positive but different, it is an ellipse.
graph{5x^2+9y^2-10x-18y-31=0 [-10, 6, -5, 5]}