% #18 
B=[ 1 3 -2 2; 0 1 1 -5; 1 2 -3 7; -2 -9 2 -1]
rB=rref(B)
% results show no pivot in last row.  Does not span

%12
A=[1 2 1 0; -3 -1 2 1;0 5 3 -1]
rA=rref(A)
% results show x1=.6 x2=-.8 x3=1 is a unique solution 