Answer: The required transformation is the REFLECTION ACROSS Y-AXIS.
Step-by-step explanation: We are given to state the type of transformation described a follows :

We note the given transformation that the sign before the x co-ordinate changes.
This transformation gives the reflection of the point across y axis.
For example, if (a, b) = (1, 2), then after reflection across y axis, the point become

These two points are plotted on the co-ordinate plane as shown in the attached figure below.
Thus, the required transformation is the REFLECTION ACROSS Y-AXIS.