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CCSS.Math:

we're told suppose suppose a plus B is equal to 2a which of these expressions equals B minus a alright I encourage you to pause the video and see if you can figure that out which of these expressions would be equal to B minus a and it's going to just involve some some algebraic manipulation all right let's let's work through this together so we are told we are told that a plus B is equal to 2a so the first thing I would want to do is get all my A's in one place and one way I could do that is I could subtract a from both sides so if I subtract a from both sides I am going to be left with just a B on the left-hand side and on the right-hand side I'm going to be left with 2a minus a well that's just going to be a if I have two of something and I subtract one of them take away one of them I'm going to have just one of those something it's going equal to 1a so we want to figure what B minus a is well luckily I can figure that out if I subtract a from both sides so if I subtract a from both sides well then I'm going to get on the left hand side B minus a which is what we want to figure out is equal to a minus a is equal to 0 so B minus a is equal to 0 which is not one of the choices all right so let's let's see if we can if we can figure out some other things over here so B minus a is equal to 0 but that is not one of the choices all right so let's see is there other other way to manipulate this now B might if I could just go straight ahead and subtract 2a from both sides and I would get B minus a is equal to 0 oh this is interesting this is a tricky one so B minus a is 0 well if B minus a is equal to 0 then if we take the negative of both sides of this if we take the negative of both sides so if we multiply both sides by negative 1 so times I should right I should write the times I should write like this right because we don't to confuse it with the variable X so if we multiply both sides by negative one what do we get well on the left hand side we get a minus B and on the right hand side we still get zero B minus a is zero then the negative of it which is a minus B is also going to be equal to zero and that's this choice that is let me do that a little darker color that is this choice right over there that was that was a good one