Answer
$7.17\times 10^5 cal$
Work Step by Step
As we know that
$1 cal=4.184J$
$\implies 1J=\frac{1}{4.184}cal$
$\implies 3\times 10^6J=\frac{3\times 10^6}{4.184}=7.17\times 10^5 cal$
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