Hello everyone!

I've been trying to solve this one problem the whole day, and even after all of the hints in the book I still can't solve it.

It says a ribbon is run around the box so that it makes a complete loop as shown in the diagram. The ribbon rests tightly against the box all way round. If the box is 60cm*30cm*15cm, how long will the ribbon be?

I have attached the diagram, and a picture of the ribbon rolled out into a straight line below.

I feel like I'm almost there, but I can never find the exact lenght for the second leg of the triangle.

Thank you in advance.