Find roots of polynomial $$ p(x) = -9x^4-3x^3 $$

The roots of polynomial $ p(x) $ are:

$$ \begin{aligned}x_1 &= 0\\[1 em]x_2 &= -\frac{ 1 }{ 3 } \end{aligned} $$

Step 1:

Factor out $ \color{blue}{ -3x^3 }$ from $ -9x^4-3x^3 $ and solve two separate equations:

$$ \begin{aligned} -9x^4-3x^3 & = 0\\[1 em] \color{blue}{ -3x^3 }\cdot ( 3x+1 ) & = 0 \\[1 em] \color{blue}{ -3x^3 = 0} ~~ \text{or} ~~ 3x+1 & = 0 \end{aligned} $$

One solution is $ \color{blue}{ x = 0 } $. Use second equation to find the remaining roots.

Step 2:

To find the last zero, solve equation $ 3x+1 = 0 $

$$ \begin{aligned} 3x+1 & = 0 \\[1 em] 3 \cdot x & = -1 \\[1 em] x & = - \frac{ 1 }{ 3 } \end{aligned} $$

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