Chapter 5 - Cumulative Test for Chapters 3-5 - Page 416: 6

$x=0$ $x=3$ $x=-7$

$f(x)=0$ $x^4+4x^3-21x^2=0$ $x^2(x^2+4x-21)=0 $ $x^2=0$ $x=0$ $x^2+4x-21=0~~$ ($a=1,b=4,c=-21$): $x=\frac{-b±\sqrt {b^2-4ac}}{2a}=\frac{-4±\sqrt {4^2-4(1)(-21)}}{2(1)}=\frac{-4±\sqrt {100}}{2}=\frac{-4±10}{2}=-2±5$ $x=3$ or $x=-7$