Denote the lengths of the sides of these $7$ equilateral triangles by $a_1, a_2, \dots, a_7$. The length of the segment $AB$ is then $a_1 + a_2 + \dots + a_7$, and the length of the polygonal curve from $A$ to $B$ is $2a_1 + 2a_2 + \dots + 2a_7 = 2(a_1 + a_2 + \dots + a_7)$. Since this length is twice the length of the segment it must be equal to $40$ cm.