These questions give you an equation that isn't linear: one with x2, a square root, x in a denominator, or an absolute value. You find the solutions, count them, find a constant such as k that controls how many there are, or reject a value that only looks like a solution.
Some questions give a system: a line and a parabola, or two curves. You solve it by substitution, count the points where the graphs meet, or read the solutions from a graph.
Some items are multiple choice with four options. Others are student-produced responses, where you type the answer into a box.
Advanced Math is about 35% of the Math section, and nonlinear equations and systems is one of its three skills. Expect to see it on test day, from quick factoring items to hard ones with a constant k.
Quadratics are the core of the skill. The discriminant, extraneous solutions and line-parabola systems all build on solving ax2+bx+c=0, so start with Lesson 1.