(%i1) algsys([z-a=%i*b,z+a=c,a*z+3*(a-z)=13-12*%i,3*(z-a)+a*z=12*%i+13],[a,c,b,z]);
(%o1) []
(%i2) algsys([z-a=%i*b,z+a=c,a*z+3*(a-z)=13-12*%i,3*(z-a)+a*z=12*%i+13],[z,a,c,b]);
(%o2) [[z=2*%i-3,a=-2*%i-3,c=-6,b=4],[z=-(3*%i+11)/(%i-3),a=3-2*%i,c=6,b=4]]
I think this is a known bug. See ID: 1944012 - solve() fails depending on equation order.
Dieter Kaiser
Diff:
Closing, as this particular example now works. Perhaps due to recent changes to algsys.