The wording "a rate proportional to the number" indicates the function is exponential. The growth factor is 1200/1000 = 1.2 in 7 days, so the exponential function can be written ...
i(t) = 1000(1.2^(t/7))
After 12 days, the number infected is modeled as ...
i(12) = 1000(1.2^(12/7)) ≈ 1367
About 1367 people are infected 12 days after discovery.
