Area of a square is 6.25 cm (square). Find each side of the square in mm.

You have mixed units... lets make every thing millimeters. So the area of your square is 625 sq. mm.

A square has 4 equal sides, lets call these equal sides length A. 

and the formula to find the area of a square is

area of a square = sidelength squared.

so

625 sq mm = A^2

so... what number when squared equals 625?

or, the same problem reversed is ... what is the square root of 625?

I'll leave you to finish that.