A student applies a force of 85 N to the back of a classmate who is sitting in the teacher's rolling chair. What is the acceleration of the student and chair if their
mass is 75 kg?

The leading coefficients are opposites.

For instance
f(x) = 2x^2 + 5x + 6
g(x) = -2x^2 + 8x - 3
here the leading coefficients 2 and -2 are opposites

Adding function f(x) and g(x) gets us
f(x) + g(x) = (2x^2+5x+6) + (-2x^2+8x-3)
f(x) + g(x) = (2x^2-2x^2) + (5x+8x) + (6-3)
f(x) + g(x) = 0x^2 + 13x + 3
f(x) + g(x) = 13x + 3

The two quadratics add up to a linear equation