Given-Find: Undefined Variable

Hello, I am working on some Laplace Transform problems and the problem got messy quickly and I figured I would just use a given find to solve for A, B, C, and D(for the partial fraction). But I am unfortunately running into an issue that I have seen before but I cannot remember how I resolved it. The reoccuring issue is the last variable in the matrix will appear to be undefined when it is clearly defined beforehand. I want to make sure to write down how to properly use the given find so this issue will not occur again. Any help would be greatly appreciated.