I have results but I don't understand some parts so if someone would like to help me, I would be thankful.

First they use

**Ampere's law:**

\(B_1\delta _1+B_2\delta _2=\mu_0N_1I_1\)

I don't understand this at all, why this sum, why those formulas?

Than

**Gauss's law:**

\(\Phi =B_1A_1=B_2A_S\)

I understand this part.

And than they use:

\(\Phi_{v2} = -\Phi N_2\)

From where comes this minus?