(a) Let A(x) denote the number (in hundreds) of computers sold when x thousand dollars is spent on advertising. Represent the following statement by equations involving A or A′: When $8000 is spent on advertising, the number of computers sold is 1200 and is rising at the rate of 50 computers for each $1000 spent on advertising.

(b) Estimate the number of computers that will be sold if $9000 is spent on advertising.