Like terms have the same variable parts.
Observe the four options 4x, -x are like terms
Option is right choice.
![\"\"](\"/userfiles/image/steps_images/step1.JPG\")
![\"\"](\"/userfiles/image/steps_images/solution.JPG\")
![\"\"](\"/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=1fee04f379fee65c8b7a2ca6bc31b27e.png\")
is right choice.