Question

If diameter of iron pipe is 2.8cm
and 1mm thickness is melted and cylindrical rod of same length is formed find diameter of rod

Answer 1

This could be a really tough question (with some x values) if you don't know the length of the original iron pipe...
Do you?
If not you could work with the assumption of a 1 m pipe.

Answer 2

depending on whether the rod's inside or outside diameter is 28mm, the cross-section of iron area is either

π(28^2-27^2) = 55π cm^2
or
π(29^2-28^2) = 57π cm^2

So, the radius of the solid rod (πr^2) is either √55 or √57 cm