A: Yes, it's a function. No input value yields more than one output.
B: If f(x) = 5x - 21, then
f(11) = 5 (11) - 21 = 55 - 21 = 34
This relation has a greater value when x=11 than the one in the table,
which lists a value of 8 when x=11.
C: f(x) =
5x - 21 = 99
5x = 120 [adding 21 to each side]
x = 24 [dividing each side by 5]