There is a square wire frame ABCD of side equal to b and whose centre is at a distance OP=b2  from an infinite line charge (parallel to BC and AD) with linear charge density λ  then [plane of the frame is perpendicular to line OP]

Imagine a cube of side b , with infinite line charge passing through its center.Since, electric field is parallel to top and bottom faces, flux passing through them will be zero.ϕcube=qenclosedε0=λbε0∴ϕeach face=λb4ε0All points on line BC are equidistant from line charge , hence electric field is constant.