a) 6! *13- 5!*12=(1*2*3*4*5*6)*13-(1*2*3*4*5)*12=720*13-120*12=
=120*(6*13-12)=120*66=7920
b)5!-1!-2!-3!-4!=1*2*3*4*5-1-1*2-1*2*3-1*2*3*4=120-1-2-6-24=87
c)7!-6!+5!-4!
=(1*2*3*4*5*6*7)-(1*2*3*4*5*6)-(1*2*3*4*5)-(1*2*3*4)=
=(1*2*3*4)*( 5*6*7 - 5*6 - 5 -1)=(1*2*3*4)*( 5*6*7 - 5*6 - 5 -1)=
= 24*174=4176