well...
if you go ahead and graph these points they appear to all be on a line.
So take a couple of them and figure out what the slope and intercept is and then double check that they all do lie on that line.
I get a slope of 3 and an intercept of -9 so the line the points lie on is
y=3x - 9
Checking this we get
1 -> -6
8 -> 15
10 -> 21
-20 -> -69
25 -> 66
and you can see that all the points lie on this line.