Why do we need to rewrite subtraction as addition when we are doing algebra

An automated water dispenser fills packets with one liter of water on average, with a standard deviation of 5 milliliter. the manual says that after a year of operation the dispenser should be tested to see if it needs recalibration. a year later a number of filled packets are set aside and measured separately. it is found that the average packet now contains about 0.995 liters. does the dispenser need calibration? explain your answer.

Is it greater than or less than7*3_9*2

Type the correct answer in each box. use numerals instead of words. if necessary, use / fir the fraction bar(s). the graph represents the piecewise function: h