Since there is no image given, the perimeter is the sum of measurement of the sides of the triangle. Assuming that the triangle is equilateral. Then the perimeter of the triangle is equal to
P = x + x + x
P = 3x
P = 30 mm
Perimeter of Rectangle: 2W + 2L
One L is the river so we have 3 sides
420m/3 = 140m
Area of Rectangle is WL
140×140= 19,600 Sq meters
I'm assuming you may have another mathematical system the teacher wants you to use. Yet, I get Area is 19,600 Sq meters
I believe it’s b- it might not be correct I believe