Answer: (x,y) → (-x,-y) 
Step-by-step explanation: 
In geometry , a rotation is an transformation of a shape by rotating it with a certain number of degree about a particular point which is called the center of rotation , and is represented by 
 ,where x is the angle of rotation about a center point o.
 ,where x is the angle of rotation about a center point o. 
Now consider the preimage of triangle ABC be (x,y) and for angle of rotation for rotating triangle ABC to A'B'C' is 180° or -180° about center of rotation(0,0).
i.e. 

 the image formed is (-x,-y).
In figure we can see the image of triangle ABC( In first quadrant ) is triangle A'B'C in third quadrant with center of rotation origin (0,0).