Answer
+3 Wb
Work Step by Step
Gauss's law states that the net magnetic flux through any closed Gaussian surface is 0. Therefore, in this case
$\Phi_{B}=\Phi_{1}+\Phi_{2}+\Phi_{3}+\Phi_{4}+\Phi_{5}+\Phi_{6}=0$
$\implies0= -1\,Wb+2\,Wb+(-3\,Wb)+4\,Wb+(-5\,Wb)+\Phi_{6}$
Or $0=-3\,Wb+\Phi_{6}$
$\implies \Phi_{6}=+3\,Wb$
