To make a reflection over the y-axis of any point on the coordinate system, what you have to do is change the sign of the x-coordinate of the point, leaving the y-coordinate the same, following the rule:
Triangle DEF was reflected over the y-axis to triangle D'E'F'
The coordinates of point F are F(6,-4)
Reflect the coordinates of F using the rule:
The coordinates of F' are (-6,-4)