You might think that you would subtract but you are really just trying to find the space between them. Think of a thermometer and draw one if it helps. Mark 90 °F about where it would be and then mark -2°F remembering to put it below 0. Now find the distance between them by adding. The distance between 90 and -2 is 92.
Is the greatest factor that two or more numbers have in common. One way to find the GCF is to make lists of the factors for two numbers and then choose the greatest factor that the two factors have in common.