Find the magnitude and direction of the magnetic induction of vector B

 a) of an infinte plane carrying a current of linear density  i, the vector i is same at all points of the plane

 b) of two parallel infinte planes carrying a current of linear density  i and -i, both the vectors being the same at all points of the plane

 

This question will be done thru ampere's law, but how to make the amperean loop???