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[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?

The scatter plot shows the number of animal cells clara examined in a laboratory in different months: plot ordered pairs 1, 20 and 2, 60 and 3,100 and 4, 140 and 5, 180 what is the approximate predicted number of animal cells clara examined in the 9th month?

Iknow the answer but the calculator doesn't seem to give the right one