What is the justification for the step taken from line 2 to line 3? Line 1: 3 x plus 9 minus 7 x is equal to x plus 10 plus x Line 2: negative 4 x plus 9 is equal to 2 x plus 10 Line 3: negative 6 x plus 9 is equal to 10 Line 4: negative 6 x is equal to 1 Line 5: x is equal to negative 1 sixth