Part B: I think the relation that has the greater value when x=6 is  f(x) = 7x - 15 because: 
7(6) - 15 = 27 which is greater than the output of 14. 
Part C: the value of x would be 3 because: 
f(x) = 7x - 15
    6 = 7x - 15
+ 15         +15
21 = 7x 
3 = x 
Sorry if any of this is wrong.