There are 4 terms. Your <em>trinomial</em> could consist of <em>any three</em> of them. Here, we've chosen to use the one with the negative sign as the monomial, because the question asks for a difference.
bisector. Base. In geometry, the base of a shape is the side (usually the bottom) that forms a right (90 degree) angle with the height of the object. Biannual.
