... I still can't get it.
You only need to know the length of one side of the blue triangle, and at least two angles for the blue triangle.
The length of the lower right side of the blue triangle isn't hard to get. It's part of another triangle with one 135 degree angle and two 30 cm sides, just to the right of it. So now you can get the length of that side.
now look at the bottom angle of the blue triangle. It's a portion of the 135 degree angle that comprises the bottom of the whole figure. Since that vertical line bisects the figure, then the blue triangle and little white triangle we just used are half of 135, or 67.5 degrees. Since we just used the little white triangle and have all the side lengths and angles for it, we know that it takes 22.5 degrees of the 67.5 degrees we have available for the bottom right half of the figure. This leave the bottom of the blue triangle with an angle of 45 degrees.
The top right line of the blue triangle is parallel to two of the faces of the figure. Let's look at the top right face. We know that it has an angle of 67.5 degrees from the vertical line. Since the top line of the blue triangle is parallel, then we know that it has an angle of 67.5 from the left side of the blue triangle.
We now know how to get the length of one side, and two angles of the blue triangle. The rest is relatively simple trigonometry.