W(x)=ax^3-x^2+3x-1
r=\frac{1}{3}
a *(\frac{1}{3})^3-(\frac{1}{3})^2+3 * \frac{1}{3}-1=0
\frac{1}{27}a-\frac{1}{9}+1-1=0
\frac{1}{27}a=\frac{1}{9} / *27
a=3