If you divide each diagonals by 2, you get the sides of the right triangle that makes the rhombus. The hypothenuses are the sides of the rhombus.
Apply the Pithagorean Theorem
Side = √(KM/2)² + (JL/2)² = √(10/2)² + (24/2)² = √5² + 12² = √25+144 = √169 = 13 (cm?)
P = Side*4 = 13*4 = 52 (cm?)