Since the pentagon is a regular pentagon, all 5 of its sides are equal, so
each side is the perimeter divided by 5, and 15 mm divided by 5 is 3 mm.

In a right triangle with a hypotenuse of 5mm and a leg of 3mm, the other
leg is 




Solve that for a.

Since the triangle is a right triangle, we may consider one leg as the base
and the other leg as the height,



Substitute in that and solve for A

Edwin

Since the pentagon is regular, its side length is    of its perimeter, i.e. 3 mm.


Having the hypotenuse of 5 mm, the given right angled triangle is  (3-4-5) triangle; hence, its other leg is 4 mm.


Then the area of this triangle is   = 6 square millimeters.


ANSWER.  The answer is B).