since the triangle is equilateral meaning all side of ABC is equal to each other AB= BC = AC= 8cm and MN is the middle line that divides AB and BC by two
thus, MB =BN= 4 cm. MN is the middle line thus will also be equal to the half of AC meaning MN=AC/2= 4cm. and perimeter is MBN = MN+MB+BN= 4cm+4cm+4cm =12cm
