What type of transformation would
change (-6,3) into (6,3)?

Reflection

Step-by-step explanation:

From the coordinates, the x variable changes its sign without having to change its y variable. So translation is impossible here.

Rotation would also be impossible to achieve this since it the coordinates would switch.

A reflexion across the y-axis

point (x , y) reflected across the y-axis is point (-x , y)

i hope this is what you were looking for.

see attached graph