Answer
$x\in \{-2, 6\}$
Work Step by Step
$3^{x^2-12}=9^{2x},$
$3^{x^2-12}=3^{4x},$
$x^2-12=4x,$
$x^2-4x-12=0,$
$x^2+2x-6x-12=0,$
$x(x+2)-6(x+2)=0,$
$(x-6)(x+2)=0,$
$x\in \{-2, 6\}$
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