Explanation:
a rhombus is a special parallelogram, as all 4 sides are equally long (as the graphic shows).
the area of a rhombus is as the area of any parallelogram :
baseline × height
we know the perimeter of the rhombus : 24 cm
that means that's the way around the whole shape one time. since all 4 sides are equally long, that means that the perimeter is equal to 4 times one side :
24 = 4 × side
side = 24/4 = 6 cm
so, we know that each side is 6 cm long. that includes the side we have the height of the rhombus for.
and the area of the rhombus is therefore
area = 6 × 3 = 18 cm²