Answer:

21, 17, 13, 9, 5, 1, -3

d=-4

Step-by-step explanation:

-solve for d

d=a5-a4

d=5-9

d=-4

-solve for a1

a2=a1+(2-1)-4

17=a1+(-4)

17+4=a1 (why +4? transpose - to +)

21=a1

a1=21

-solve for a3 and a7

a3=21+(3-1)-4

a3=21+(2)-4

a3= 21+(-8)

a3=13

a7=21+(7-1)-4

a7=21+(6)-4

a7=21+(-24)

a7= -3