In this case the answer is very simple .
Step 01:
Data
A (-1,1) B (1,3) C (4,1)
Step 02:
reflection y = x
(x , y ) ====> (-x , -y )
A' ( 1 , -1 )
B' (-1 , -3)
C' (-4 , -1)
Those would be the new coordinates of the triangle.
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