Let’s label the triangles A, B , C
Since the triangles are equilateral triangles , each triangle will have equal length for its 3 sides
perimeter of A = a + a+ a
perimeter of B = b + b+ b
perimeter of C = c + c + c
the total length would be the length of all triangles .
since it is given that a + b + c = 11cm
3 a + 3b + 3c = 11 x 3