For quadrilateral ABCD to be a rectangle, its diagonals AC and BD must be equal in length. Therefore, we can set up the equation:
\[ AC = BD \]
Substituting the expressions given:
\[ x + 7 = 3x - 5 \]
Now, we'll solve for \( x \):
- Subtract \( x \) from both sides:
\[ 7 = 2x - 5 \]
- Add 5 to both sides:
\[ 12 = 2x \]
- Divide both sides by 2:
\[ x = 6 \]
Thus, the value of \( x \) that makes ABCD a rectangle is \( x = 6 \).
So the correct response is: x = 6.