Question

a concave lens of suitable focal length is used to correct?

Answer 1

Hyper metro Pia is the answer I think

Answer 2

The concave lens of suitable focal length used to correct myopia because the concave lens diverges the light before it reaches the eye. This light is then converged by the cornea and eye lens to produce an image on the retina.
Since, we require to diverge the light before reaching the eye to correct myopia, we cannot use a convex lens in place of the concave lens.