Of these three linear functions represented below, what has the greatest rate of change?

-function 1: y=2x+6

-function 2: (this is a table)
x|y
-1|3
1|15
2|21

-function 3: a number y is less than half a number x.

a) function 3 has greatest rate of change
b) function 2 has the greatest rate of change
c) function 1 has the greatest rate of change
d) all three functions have the same rate of change
*would like the steps to solve this also.