This is a dose-response experiment testing a new cancer drug. the darker line represents cancer cells. what range of dose should be used? I think it's 2-4 because this affects cancer cells only. is this correct?
In the most basic sense you want to kill the most cancerous cells whilst minimizing the regular somatic cell death. Almost all cancer medications affect regular cells, too - though the better ones do so at a minimum whilst being effective. In reality, it's also nearly impossible to kill all of the cancerous cells. The goal is to bring them below detectable levels, which can allow the body to finish the job. Leaving significant amounts of cancerous cells alive won't do the patient any good - they'll just continue to proliferate and the patient will be back for more operations or treatments soon.
So, with the goal of minimizing benign cell cost and completely eradicating the cancerous line, on your crude chart that falls at about "4".