Answer
$9a^{13}$
Work Step by Step
$(2a^{3})^{3}a^{4}+a^{5}a^{8}=2^{3}a^{3\times3}a^{4}+a^{5}a^{8}=8a^{9}a^{4}+a^{5}a^{8}=8a^{9+4}+a^{5+8}=8a^{13}+a^{13}=9a^{13}$
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