The solution is known to be equivalent to $\bigwedge_i (a_i>0) \land \bigwedge_j (b_j>0) \land \sum_i a_i=\sum_j b_j$.
load redlog; rlset ofsf;
mtp2 := rlex(
x11+x12+x13=a1 and x21+x22+x23=a2 and x31+x32+x33=a3 and
x11+x21+x31=b1 and x12+x22+x32=b2 and x13+x23+x33=b3 and
0 <= x11 and 0 <= x12 and 0 <= x13 and 0 <= x21 and
0 <= x22 and 0 <= x23 and 0 <= x31 and 0 <= x32 and 0 <= x33,
{a1,a2,a3,b1,b2,b3});
mtp2sol:= rlqe mtp2$
rlatnum mtp2sol;
end; % of file