When you're looking for the changes between the two, it is best to simply look for the differences. In this case, the first equation has a positive 4 included, whereas, the second doesn't. When you've got a positive number, you always subtract it away from the otherside, as you always do the opposite to what the operation is.This is the equation solved. [tex]x+3x= 72-4 \\ 4x=68 \\ x= 17[/tex]
Hope this helps :)
