Good luck, and may your determinants be non-zero.

For students enrolled in the Kumon Mathematics program, few milestones inspire as much respect—and perhaps a bit of dread—as reaching Level O . This level represents the gateway from high school calculus into the uncharted waters of introductory college mathematics. Specifically, Level O introduces Differential Equations, Vector Calculus, and Linear Algebra .

"This is not separable. It looks homogeneous, but I forgot the substitution."

You cannot succeed in Level O without a feedback loop. The Kumon answer book provides that feedback. However, relying on it entirely is like using training wheels on a unicycle—it defeats the purpose. Use the solution book to understand why you were wrong, not to find what is right.