In order to determine the side length of the parallelogram. Make sure that you know, the fact that the 2 bottom sides are parallel and the 2 longer portions are parallel.
To find he value of X, then simply make the 2 sides of the same length of the rhombus equal to each other and solve for x.
4x + 3 = 15
4x = 12
X = 3.
To find the length of the other sides, simply substitute, 3 for the other side.
3x
3(3) = 9.
Both of the those sides are 9 cm. I believe these would be the solution.