To determine the value of \( x \) that makes quadrilateral \( ABCD \) a rectangle, we need to know that in a rectangle, the diagonals are equal in length. This means we can set the lengths of the diagonals equal to each other:
\[ AC = BD \]
Given: \[ AC = x + 7 \] \[ BD = 3x - 5 \]
Setting these equal gives us the equation:
\[ x + 7 = 3x - 5 \]
Now, we can solve for \( x \):
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Subtract \( x \) from both sides: \[ 7 = 2x - 5 \]
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Add 5 to both sides: \[ 12 = 2x \]
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Divide both sides by 2: \[ x = 6 \]
The value of \( x \) that makes \( ABCD \) a rectangle is \( x = 6 \).
Thus, the correct answer is: x equals 6.