Answer:
y = x
Step-by-step explanation:
Let A(-1,-4) and B(-4,-1).
The slope of AB is -1, and the midpoint of AB is (-2.5, -2.5).
The perpendicular bisector of AB thus has a slope of 1 and passes through (-2.5, -2.5).
The perpendicular bisector is the line of reflection, which has equation y = x.