linear means that it is 1st degree, means it has x¹ basically the highest number is 1 for the power of the placeholder
quadratic means it is 2nd degree, means it has x² in it
so expand and simlfiy
(x-5)(5x+4)-5x²=
5x²-21x-20-5x=
-21x-20
so y=-21x¹-20
it's linear because the squared terms cancel each other out
