Problem
Tracing the outline, the border is 14 square sides; the inner fold lines are not part of it.
Perimeter means the distance around the outside, so I only count the sides on the border.
3.MD.D.8Draw A DiagramThe outline of the net is made of 14 square sides, every one of them a cube edge long.
Walking round the outside, only the square sides that have no neighbour attached lie on the boundary; the sides where two squares meet are tucked inside and never on the outline.
All six squares are the faces of one cube, so every one of those boundary pieces is the same single edge length, and 14 of them make the whole way round.
All 14 border pieces are the same length, so one edge length is 210 cm split into 14 equal parts.
When equal pieces add up to a total, dividing the total by the number of pieces gives one piece.
3.OA.A.3Identify SubproblemsCount the edges around the outline, then share the perimeter equally among them to find one edge.