A scale factor of 1/2 makes the triangle half the size of the original.
The original is 4 units tall, so 1/2 scale makes the new one 2 units tall.
It is 6 units long, so 1/2 would be 3 units long.
Now since the scale factor is centered at (-1,3) fine the midpoint between The top left corner of the triangle and the center point:
Midpoint ( -1-5/2 , 3+7/2)
Midpoint = (-6/2 , 10/2)
Midpoint = -3,5
This is where the top left corner of the scaled triangle would be.
See attached picture: