4x - x
If there is a single variable with no number in front you can assume that there is a 1. How I think of it is that all singular variables have an invisible one in front
so...
4x - 1x
Since these are like terms you may subtract them like normal numbers. The only difference is that you'll have to add an x at the end of your answer
4 - 1
3
(REMEMBER to add the x back to the three)
so...
4x - x = 3x
Hope this helped!
~Just a girl in love with Shawn Mendes