6см 1мм is equal to 60.1mm.
40мм is equal to 4см.

So, the total length is:
60.1mm + 4см = 64.1mm + 4см = 64.1mm + 0.4 = 64.5mm

5дм is equal to 50см.
12см is equal to 1дм 2см.

So, the total length is:
50см + 1дм 2см = 50см + 10см + 2см = 60см + 2см = 62см

Therefore, the combined length is 64.5mm + 62см = 64.5mm + 620мм = 684.5mm.