Since the "x"'s match you can put them together! so (-2x)-(x)= -3x . When we add/subtract variables like "x" we only subtract/add the numbers in front of the variables and also, a variable by itself is just representing 1 of that variable that's why we can say that x means 1x and we can subtract -2 by 1. Now we are done because there are no more like numbers left we can just leave the expression as --> -3x+8
