Verified answer

Sn= (a1+an)*n/2

Sn= (2a1+d(n-1))*n/2

-36= (a1-15)*n/2

-72= n(a1-15)

-36= (2a1-3(n-1))*n/2

-72=n(2a1-3(n-1))

a1= -72/n+15

-72= n(2(-72/n + 15)-3(n-1))

-72=n(-144/n+30-3n+3)

-72= n(-144/n-3n+33)

-72= -144-3n²+33n

n>0

3n²-33n+72=0

n²-11n+24=0

D= 121-96= 25

n1= (11+5)/2= 8

n2= (11-5)/2= 3

если n1= 8, то a1= 6

если n2= 3, то a1= -9