If a quadrilateral is a rhombus then:
i) all its sides have equal length,
and
ii) the 2 diagonals are perpendicular and bisect each other.
The 2 diagonals of rhombus JKLM intersect at N.
Then m(MNJ)=90°, |JN|=4 and |NM|=3 units
thus, by the Pythagorean theorem,

The perimeter of the rhombus is 4*5 units=20 units.
Answer: 20 units