Well, he is basically saying that the set-up for finding the distance between a number, a, and another number, b, is |a-b|=distance. With that in mind, if you want the set-up for numbers that are a distance of 4 or less from the number -5, it would look something like |x-(-5)|<=4 or |(-5)-x|<=4. If you simplify the first one, you get |x+5|<=4. Do you see how all that is happening is substituting a for x and b for -5 in the setup |a-b|=distance?
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this explanation makes no sense
Well, he is basically saying that the set-up for finding the distance between a number, a, and another number, b, is |a-b|=distance. With that in mind, if you want the set-up for numbers that are a distance of 4 or less from the number -5, it would look something like |x-(-5)|<=4 or |(-5)-x|<=4. If you simplify the first one, you get |x+5|<=4. Do you see how all that is happening is substituting a for x and b for -5 in the setup |a-b|=distance?