Question

Which transformation would change the point (5,3) into (-5,3)?

Answer 1

Assuming it is on a grid map, (5,3) would be in the 1st quadrant. and (-5,3) would be in the 2nd quadrant. You would have to move the coordinate 10 spaces to the left. 

Answer 2

A transformation like this would be a reflection across the Y access. If you flip this point about the Y access, the Y will not change but you will get the inverse of the X.
Hope this helps! :D