Answer:
![r_(y-axis)(x,y)\rightarrow(-x,y)](https://img.qammunity.org/2019/formulas/mathematics/middle-school/xgtwloo8ymrn2bsz1fb4yg7xvfs6wj6r7f.png)
Explanation:
From the given picture, it cam be seen that the two triangles are reflection of each other.
Such that their corresponding x coordinate has different sign but same magnitude and the corresponding y-coordinate remains unchanged.
Thus, the reflection must be reflection across y-axis.
The rule of the reflection across y-axis is given by:-
![r_(y-axis)(x,y)\rightarrow(-x,y)](https://img.qammunity.org/2019/formulas/mathematics/middle-school/xgtwloo8ymrn2bsz1fb4yg7xvfs6wj6r7f.png)