Verified answer

cn = c1 + (n-1)d

c2 = c1 + d   d= c2 - c1

c5 = c1 + 4d

c10 = c1 + 9d

c10 - c5 = 5d

c10 - c5 = 4.7 - 8.2 = -3.5

5d = -3.5

d = -0.7

c5 = c1 + 4d

8.2 = c1 + 4*(-0.7)

c1 = 8.2 + 2.8 = 11