There’s size sides in total in this picture. We are given four; we must find the other two.

10.6 + 10.3 + 3.9 + 2.9 + x + y = z

x and y are unknown, and z is the answer you want to reach.

Notice that for the VERTICAL unknown length (I’ll call it y), you can get this value by subtracting 10.6 with 3.9.
10.6 – 3.9 = 6.7
This is y = 6.7

Notice that for the HORIZONTAL unknown length (I’ll call it x), you can get this value by subtracting 10.3 with 2.9.
10.3 – 2.9 = 7.4
This is x = 7.4

Let’s go back to the first equation at the very top and plug in x and y.

10.6 + 10.3 + 3.9 + 2.9 + 7.4 + 6.7 = 41.8

The answer is 41.8 meters.

(There was actually an easier way to do this, but it would only work in this specific picture, so I decided not to cover it.)