Two quantities are related, as shown in the table:

x y
2 10
4 9
6 8
8 7

which equation best represents the relationship?

OK here is it 
y=mx+c is the basic equation for a linear graph 

If u plot the values above on a graph u'll get a straight line so 
 m is the slope or if they havent taught u slope is amount of y increases or decreases with respect to x. and C is the intercept or in other words where the graph or the system given above meets the y axis.

the above info is quite unnecessary but important to know 
so putting the values into the equation
Youll get the values of m and C so

m=-1/2 and C=11 

the relation between x and y is
y=(-1/2)x+11

put the values from the table ull get the right answer 

C. i possibility the answer to this