Answer:
31
Explanation:
For rectangle ABCD, AC and BD are the lengths of the diagonals. The diagonals of a rectangle are congruent.
AC = BD
3x + 7 = 131 - x
Add x to both sides.
4x + 7 = 131
Subtract 7 from both sides.
4x = 124
Divide both sides by 4.
x = 31