помогите решить 6 задание

a1=1512,a2=-252,a3=42
q=a2/a1=a3/a2=.......
q=-252/1512=-1/6, q=-1/6
s4=a1. (qˇ4 -1)/(q-1)
s4=1512.(1/1296-1)/(-1/6-1)=
      =1512.(-1295/1296)/(-7/6)=
   = 1512.1295.6/1296.7=1512.1295/216.7=7.1295/7=1295
s4=1295