Question 284532
first you have to understand what linear rate means.
Linear means it make a line.
If you plot (1,6) (2,12) (3,18) will it make a line.
Try it.
the formula for a line is y=mx+b
in this  case b=0
and m =6
y=6x 
try them 
6=6*1
12=6*2
18=6*3
so yes it is linear