Answer
$\infty$
Work Step by Step
$$\lim _{x\rightarrow \infty }\left( 3x^{12}-9x^{7}\right) =\lim _{x\rightarrow \infty }x^{12}\left( 3-\dfrac {9}{x^{5}}\right) =\lim _{x\rightarrow \infty }3x^{12}=\infty $$
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