50g Multiple equation Solver (MES)

[LGVE]

I was reading about MES in the HP 50g User's Guide and tried the example of solving triangles ( page 7- 9) on my calculator. It worked fine but when I tried to solve a triangle problem, one of the sides was negative. The data:
a=20
alpha=45 degrees
betha=60 degrees

I press White key with left arrow and ALL
I press Orange key with right arrow and ALL

The calculator gives as answers:
phi=75 degrees
b=24.49
c=-7.32

c must be positive.

Any suggestions?

Thanks in advance

Luis