The equation of an equipotential line in an electric field is y=2x then the electric field strength vector at (1,2) may be?

the electric field lines are always perpendicular to the equipotential surfaces.

the electric field strength vector will be along the line x + 2y = 0.

however to know the exact representation of the field strength vector we need to know some other data.

1. the potential function in terms of x and y co-ordinates must be known.

2. otherwise if the data is given in terms of equipotential surfaces then some more equipotential surfaces must be given, their shapes and potentials must be known and the separation between them must be known so that we can get an idea about the nature of the field and the source charge.