To determine the value of \( x \) that makes the quadrilateral \( ABCD \) a rectangle, we use the property that in a rectangle, the diagonals are equal in length.
Given:
- The length of diagonal \( AC \) is \( AC = x + 7 \)
- The length of diagonal \( BD \) is \( BD = 3x - 5 \)
Since the diagonals must be equal in a rectangle, we can set the expressions for the lengths equal to each other:
\[ x + 7 = 3x - 5 \]
Now, we can solve for \( x \):
- Subtract \( x \) from both sides:
\[ 7 = 3x - x - 5 \]
This simplifies to:
\[ 7 = 2x - 5 \]
- Add \( 5 \) to both sides:
\[ 7 + 5 = 2x \]
\[ 12 = 2x \]
- Divide by \( 2 \):
\[ x = \frac{12}{2} = 6 \]
Thus, the value of \( x \) that makes \( ABCD \) a rectangle is:
\[ \boxed{6} \]