the parabola and line intersect at x = 0, and , except at x = 0,
it does not make a difference.
if f(x) > g(x), then the area between them is no matter where f and g reside relative to the x-axis.
here is a simple analogy ... consider two points (1, 5) and (1, 2) ... the distance between the two points is 5 - 2, correct?
now, what about (1,5) and (1,-2) ... isn't the distance 5 - (-2) ?
it's the same idea with the functions.