To find the value of \( x \) in the rectangle \( ABCD \) where \( CO = -3x \) and \( BD = x + 21 \), we can take advantage of the properties of rectangles.
In a rectangle, the diagonals are equal in length. Hence, we can set the lengths of the diagonals \( CO \) and \( BD \) equal to each other:
\[ -3x = x + 21 \]
Now, let's solve for \( x \):
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Add \( 3x \) to both sides: \[ 0 = 4x + 21 \]
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Subtract \( 21 \) from both sides: \[ -21 = 4x \]
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Divide both sides by \( 4 \): \[ x = -\frac{21}{4} \]
Therefore, the value of \( x \) is
\[ x = -\frac{21}{4} \]
Thus, the correct answer is:
x equals negative Start Fraction 21 over 4 End Fraction.