Find a number M such that $\mid x^3 - x^2 +8x \mid\leq M$ for all $-2\leq x \leq 10$ Find a number M such that $\mid x^3 - x^2 +8x\mid \leq M$ for all $-2\leq x \leq 10$

I am not sure my answer is right or not.. 

I used triangle inequality. 

$\mid x^3 - x^2 +8x\mid \leq \mid x^3 \mid + \mid x^2  -8x \mid  \leq \mid x^3 \mid + \mid x^2 \mid + \mid 8x \mid $ 

$M = 10^3 + 10^2 + 8(10)$
Isn't the best result |$ 10^3 - 10^2 +8*10$|?see the image here
Isn't the best result |$ 10^3 - 10^2 +8*10$|?see the image here