Hello I need help on it please

Rearrange the formula c^2 = a^2 + b^2 for b

On a piece of paper graph f(x)=5•(0.4)^x

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.