Calculate the observed mean, expected mean, standard deviation and z value for the median and up/down test.

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Line ab is y = -7/5x + 56/5i don't understand  where point d comes into this. 

You move the constant to the right side and change its sign to be 9z= -6 -8. that’s the first step.the second step would be calculate the difference of -8-6 to get -14. you equation should now be 9z=-14the last step is to divide both sides by 9 to get z=-14/9hope this