How to find out side of parrallelogram,if the ratio of adjacent side & area is given?

No, that won't suffice. You will need the height (perpendicular between two parallel sides) or the angle between two adjacent sides. In the case of an rectangle, which is a special case of parallelogram, you have the angle 90°. Hence the area: a * b * sin 90 = a * b * 1 = a * b.
Given the ratio 2:3 and the area this would add up to:
a * (3 / 2) * a = a² * (3 / 2) = 1536
a² = 1024
a = 32

But in the general case of a parallelogram with no given angle nor the height there is no unique solution.