well, generally the difference is the larger number minus the smaller.
For example, the difference between 5 and 12 is 12-5 = 7, not -7.
So, for this problem, I'd say that the difference is 118-(-30) = 148
Other than that, it looks good.
find the difference of an elevation of 30 feet below sea level and an elevation of 118 feet above sea level
ans.. -30 - 118 = -148
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