Why here the formula of magnetic field is not used of biot savart law

Dear Student,
The present problem is more about finding the induced current in the square loop due to a time varying current in the straight long wire. For this, the requirement is to use Faraday's law.
That is the reason, the magnetic field due to a straight long current carrying wire is straight away written as:
$$\begin{gathered} B=\frac{{\mu }_{o}I}{2\pi x} \\ where, \\ x is the dis\tan ce of the point from the wire. \\ Also, \\ a<x<2a \end{gathered}$$
Moreover, for infinite current elements, Ampere's law is an easy approximation over Biot-Savart's law. 
These must be the reason for specific not specifying magnetic field using Biot Savart law.

Regards
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