Answer
The statement does not make sense.
Work Step by Step
$D$ is the determinant of the coefficient matrix.
$D_{x}$ is obtained by replacing the 1st column in D with the constants column.
Generally, the structure of $D_{x}$ is such that its calculation very different to that of $D.$
The text does not suggest any "shortcuts" in calculating $D_{x}$ once $D$ is calculated (since there isn't any).
So, the statement does not make sense.
