2Sn= (а1+an)*n

n= 2Sn : (a1+an)

n= 2*(-170) : (1+(-35))

n = -340 : (-34)

n = 10

an = a1 + d*(n-1)

-35 = 1+d*(10-1)

-35 = 1+9d

9d = -36

d = -36 : 9

d = -4

Ответ: d = -4