[请教]XDF数学 126

IN the figure, two rectangles with the same dimensions overlap to form the shaded region. If each rectangle has perimeter 12 and the shaded region has perimeter 3, what is the total length of the heavy line?

a) 15

b) 18

c) 21

d) 22

e) 23

请问， "the total length of the heavy line"什么意思？这道题目怎么做？