The new coordinates are given by :
[Vertex of triangle - Center of Dilation ] * (scale factor) + Center of Dilation
So we have
[ ( -8, - 2 ) - ( 1, - 2 ) ] (1/3) + ( 1, -2 ) =
[ ( -9, 0 ) ] (1/3) + ( 1, - 2 ) =
( -3, 0 ) + (1, - 2 ) = ( -2, -2 )
[ ( -8, 4 ) - ( 1, - 2 ) ] (1/3) + (1, - 2) =
[ ( -9, 6) ] (1/3) + ( 1, - 2 ) =
( -3, 2 ) + ( 1, -2 ) = ( -2, 0 )
[ ( 4, 7 ) - ( 1, - 2 ) ] (1/3) + ( 1 , -2 ) =
[ ( 3 , 9 ) ] (1/3) + (1, - 2 ) =
( 1 , 3 ) + ( 1, - 2 ) = (2, 1)
Here are the pre and post images of the coordinates :
https://www.desmos.com/calculator/jahlfxvluy