#### Problem 90E

90. The equation $y^{\prime \prime}+y^{\prime}-2 y=x^{2}$ is called a differential equation because it involves an unknown function $y$ and its derivatives $y^{\prime}$ and $y^{\prime \prime}$. Find constants $A, B$, and $C$ such that the function $y=A x^{2}+B x+C$ satisfies this equation. (Differential equations will be studied in detail in Chapter $9 .$ )