For quadrilateral ABCD to be a rectangle, the lengths of the diagonals \( AC \) and \( BD \) must be equal. We have the expressions for the lengths of the diagonals:
\[ AC = x + 7 \] \[ BD = 3x - 5 \]
To find the value of \( x \) that makes the diagonals equal, we set the two expressions equal to each other:
\[ x + 7 = 3x - 5 \]
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 by 2: \[ x = 6 \]
Now, we can box the solution:
\[ \boxed{6} \]
To confirm that this value makes \( AC \) and \( BD \) equal, we can substitute \( x = 6 \) back into the expressions for \( AC \) and \( BD \):
\[ AC = 6 + 7 = 13 \] \[ BD = 3(6) - 5 = 18 - 5 = 13 \]
Since \( AC = BD = 13 \), the solution is verified. Thus, the value of \( x \) that makes ABCD a rectangle is \( \boxed{6} \).