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[Han]

1. The CAS() command requires a string argument
2. You are mixing the use of local variables with global variables (note that A is a built-in global variable but you've also created a local variable named A as well).

Try:

cascmd:="solve(" + A + "*x+" + B + "*y+" + C + "=0,y)";
CAS(cascmd);

You are essentially trying to use the CAS to do a symbolic solve. If that is the goal, why not just build a CAS program to do the solving, and use it as a sub-routine?