Is twice a number subtracted from four times a number, added to 15 linear or nonlinear?

Ineed in figuring out this problem.

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

The annual snowfall in a town has a mean of 38 inches and a standard deviation of 10 inches. last year there were 63 inches of snow. find the number of standard deviations from the mean that is, rounded to two decimal places. 0.44 standard deviations below the mean 2.50 standard deviations below the mean 0.44 standard deviations above the mean 2.50 standard deviations above the mean