[Help-glpk] two fases

[miguel montoya]

I am working with the following problem :

MAX  Z=x1+6x2-7x3+x4+5x5

Subjecto to 

5x1-4x2+13x3-2x4+x5=20
x1-x2+5x3-x4+x5=8

the obtained result when I put the problem in GLPK is:

PROBLEM HAS UNBOUNDED SOLUTION

That was necessary to work with the method of two phases. GLPK has the option to work with this method ?????

That the ideal solution of the problem is : x3=12/7   x2=4/7
all other xj=0 z=-60/7   but I not as obtaining this response
using GLPK.

That I can do to obtain the ideal solution ?????????????

thank you