Answer
$x=\frac{1}{5}$
or
$x=-1$
Work Step by Step
$f(x)=0$
$5x^2+4x-1=0~~$ ($a=5,b=4,c=-1$):
$x=\frac{-b±\sqrt {b^2-4ac}}{2a}=\frac{-4±\sqrt {4^2-4(5)(-1)}}{2(5)}=\frac{-4±\sqrt {36}}{10}=\frac{-4±6}{10}$
$x=\frac{2}{10}=\frac{1}{5}$
or
$x=-1$
