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