In [1]:
A = [18 -3   0  0.0; 
     2 -13   1  10 ;
     0   7  -17 10 ;
     0  28   4 -39 ]
b = [500.0; 0; 0; 0]
v = A \b
Out[1]:
4-element Array{Float64,1}:
 30.2876
 15.0592
 13.3672
 12.1827
In [2]:
A = [3.0 6  9; 
     2   5 -2; 
     1   3 -1]
b = [39; 3; 2.0]
A \ b
Out[2]:
3-element Array{Float64,1}:
 2.0
 1.0
 3.0