Answer
144
Work Step by Step
$S = 2*\sqrt I -9$
$15 = 2*\sqrt I -9$
$15+9 = 2*\sqrt I -9+9$
$24 = 2*\sqrt I$
$24/2 = 2*\sqrt I/2$
$12 = \sqrt I$
$12^2 = (\sqrt I)^2$
$144 = I$
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