What the value of each expression is

5-7=-2     -5+(-7)=12   -5+7=2  -5-(-7)=2

whenever there are 2 negative signs next to each other you add the numbers

even though you have attached neither the chart nor the generalizations, i have located them online. the right answer is d, since they are asking you about a generalization. you can only infer from the chart that between 2008 and 2009 the number of employed workers decreased significantly (from over 115 millions to 107 millions approximately), which signals some type of economic crisis.  

the height of the rock is 15