Well, it's an old and well known problem. If you search the Web for "+Euler +theorem +graph" you will rpobably run into a solution. But you may also think a little as well.

There are 3 boxes with 5 segments each. For each box, getting into the box and then getting out, or getting out first and then getting in, means crossing two of its segments. If a box has an odd number of segments, then you either have to start or end there. But you have 3 such. Now do the thinking. (The outside should be also looked at as a region whose boundary is composed of the segments - seven segments to be exact. So in fact you have four regions with an odd number of segments.)