Answer
$\Phi_{B 6}=+3 \mathrm{~Wb}$
Work Step by Step
We use $\sum_{n=1}^6 \Phi_{B n}=0$ to obtain
$
\Phi_{B 6}=-\sum_{n=1}^5 \Phi_{B n}\\=-(-1 \mathrm{~Wb}+2 \mathrm{~Wb}-3 \mathrm{~Wb}+4 \mathrm{~Wb}-5 \mathrm{~Wb})\\=+3 \mathrm{~Wb}
$
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.