6. (10 points) Observations of a firm's sales revenue (in $'000) per month are as follows: Month 1 2 3 4 5 6 7 8 9 10 11 12 Revenue 4.5 4.2 4.6 4.4 5.0 5.3 5.2 4.9 4.7 5.4 5.3 5.8 a. Show an expression for average monthly sales revenue for the first months and evaluate for = 4 . b. Show an expression for average monthly sales revenue over the preceding 3 months for any given month , assuming that is not less than 4. Evaluate for = 10 .