To determine the value of \( x \) that makes quadrilateral \( ABCD \) a rectangle, we need to use the property that the diagonals of a rectangle are equal in length.
Given:
- Length of diagonal \( AC = x + 7 \)
- Length of diagonal \( BD = 3x - 5 \)
Setting the two lengths equal, we have:
\[ x + 7 = 3x - 5 \]
Now, we will solve for \( x \).
- First, subtract \( x \) from both sides:
\[ 7 = 2x - 5 \]
- Next, add 5 to both sides:
\[ 12 = 2x \]
- Now, divide both sides by 2:
\[ x = 6 \]
Thus, the value of \( x \) that makes \( ABCD \) a rectangle is:
\[ \boxed{6} \]