Answer
$x = 5$
Work Step by Step
The determinant is:
$D = 1 \times 7 - x \times 2$
and we know that $D=-3$
Therefore
$1 \times 7 - x \times 2 = -3$
$7 - 2x = -3$
$-2x = -10$
$x = 5$
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