Since all these lines are tangents: the unknown length along BC is also equal to 3.3 units. The first segment from C going towards D is also 5.1, meaning that the segment on CD closer to D is 7 units. This is the same length of the first segment from D going towards A, meaning that the segment on DA that is nearest A is 5.2 units long, which is also the length of the unknown segment along AB.
Therefore the perimeter is: 12.2 + 12.1 + 5.1 + 3.3 + 3.3 + 5.2 = 41.2 units