Answer
$12$
Work Step by Step
Using order of operations, the given expression, $
3[42-2(10^2-9^2)]
,$ simplifies to
\begin{array}{l}\require{cancel}
3[42-2(100-81)]
\\\\=
3[42-2(19)]
\\\\=
3[42-38]
\\\\=
3[4]
\\\\=
12
.\end{array}
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