I dont get this question could someone help me out i will mark brainliest include steps!

Answer:
12.5
Step-by-step explanation:
In a rectangle, the two diagonals are equal length, so MK = JL = 2x + 9
Note MK = 2 * MN, so
2x + 9 = 2 * (3x + 1)
expand:
2x + 9 = 6x + 2
Solve:
9 - 2 = 6x - 2x
4x = 7
x = 7/4
MK = JL = 2x + 9 = 2 * 7/4 + 9 = 7/2 + 9 = 3.5 + 9 = 12.5