Recall that

where
is the angle between the vectors
whose magnitudes are
, respectively.
We have


Well you find how much the error was and divide it by the amount he predicted.
30/230= 0.13 0.13=13%
The domain is the x-values used by the graph, so you want to see how your graph lines up with respect to the x-axis.
This graph starts out with an x-value of 2 (although it doesn't really use 2, since it's an open circle) and then continues to the right forever.
The domain is x > –2.