6. iPhone Sales iPhone sales from the 2nd quarter in 2007 through the 2nd quarter in 2008 can be approximated by S(t) = -410t2 + 3600t - 4800 thousand phones (2 s t s 6) in quarter t. (t = 1 represents the start of the first quarter of 2007.) iPhone sales (thousands) 2.000 1.500 000'I 500 0 2 3 5 2007 Q2 2008 Q2 a. Compute S'(t). How fast were iPhones sales changing in the third quarter of 2007 (t = 3)? (Give the correct unit of measurements.)