50g

hi, im having some basic problems, i just bought my 50g but there are
some basic thing i dont know how to do, like solving systems of
equations of 3 unknowns, like:
 3a-2b+1c=4
 a+4b-5c=7
 5a-6b-9c=19

for that kind of equations im still using my casio calculator =( , so
i hope you can help me, thanks!