What is the justification for the step taken from line 2 to line 3?

5 lines of math. the first line has, 3 x plus 9 minus 7 x equals x plus 10 plus x. the second line has, negative 4 x plus 9 equals 2 x plus 10. an arrow pointing from the front of the second line to the front of the third line. the third line has, negative 6 x plus 9 equals 10. the fourth line has, negative 6 x equals 1. the last line has x equals negative startfraction 1 over 6 endfraction.