Question 1151424
2x becomes 2(-x) or -2x. That only works when x=0, so in general it does not work
5x becomes -5x, and that is not x
x can be exchanged for -x by definition, for it is given
x+x+x can be exchanged for x+x+-x, since x can be exchanged for -x
2x+x=3x, so this exchange works.  It would also work for 2x+ -x, although that is not asked.