# Regarding the use of Ampere's Law

i wanted to calculate $$\mathbf{B}$$ of these two parallel wires, and i've taken a glance at the correction and i've found out this equation, i tried to understand it but in vain, where do $$(x + \frac{d}{2})$$ and $$(x - \frac{d}{2})$$ come from? also why is the infinitesimal element is the vector product of $$(\mathbf{e}_z \times \mathbf{e}_x)$$ and not $$(\mathbf{e}_x \times \mathbf{e}_z)$$?

Any help even an advice is extremely appreciated. I can link the whole assignement for more clarification. Many thanks.