As online attacks persist, spending on information security software continues to rise. The following table gives the forecast for the worldwide sales (in billions of dollars) of information security software through 2007 (x = 0 corresponds to 2002).

y = 10^9 (6.87 + 2.1 (year-2002))

Step-by-step explanation:

y = a + b x

x mean = 15/6 = 2.5

y mean = 72.7/6 = 12.12

x-x mean

-2.5 - 1.5 -.5 +.5 + 1.5 + 2.5

y - y mean

-6.12 - 3.22 -.22 + 1.88 + 3.58 + 4.08

(x - x mean) (y - y mean)

15.3 4.83.11.94 5.37 10.2

sum of those = 36.75

(x - x mean) ^2

6.25 2.25.25.25 2.25 6.25

sum of those = 17.5

so

b = 36.75/17.5 = 2.1 agree with you

a = y mean - b x mean

= 12.12 - 2.1 * 2.5 = 6.87 agree with you

y = 6.87 + 2.1 t

perhaps you used x in last line instead of t?

or

perhaps they want real years and billions

y = 10^9 (6.87 + 2.1 (year-2002))