now write the problem in the special format (missed terms are written with zero coefficients):
[ATTACH=full]3901[/ATTACH]
[ATTACH=full]3902[/ATTACH]
[ATTACH=full]3904[/ATTACH] then you have
[ATTACH=full]3903[/ATTACH]
so
[ATTACH=full]3905[/ATTACH]
[ATTACH=full]3906[/ATTACH]
[ATTACH=full]3907[/ATTACH]
