This paper has two parts. First, we propose a method which can be used to decompose a geometric constraint graph into a c-tree. With this decomposition, solving for a well-constrained problem is reduced to the solving for smaller rigids if possible. Second, we give the analytical solutions to one of the basic merge patterns used to solve a c-tree: the 3A3D general Stewart platform, which is to determine the position of a rigid relative to another rigid when we know three angular and three distance constraints between the two rigids. Keywords. Geometric constraint solving, parametric CAD, general construction sequence, decomposition tree, generalized Stewart platform, assembly.