Part 1.) This is indeed a function. A function means that there no x values have multiple (different) values
For example:
x y
1 4
2 6 <-- First x
3 8
2 10 <-- Different y-val from first x (2)
5 12
Part 2.) I'm assuming the relation is actually f(x) = 2x + 13. If so, input 7 as x.
f(7) = 27.
27 > 8.
Second relation has greater value when x = 7
Part 3.) Work backwards using this information
75 = 2x + 13
62 = 2x
31 = x <-- x is equal to 31