at right is graph of this situation predict how the line would change to present the average cost of gas in dicember of 2005 when gas cost $2.20 per gallon on average

The average cost represents the slope of the line of the graph.

Increasing the slope of the graph increases the steepness of the line.

Therefore, increasing the average cost of gas from $1.50 to $2.20 will increases the steepness of the line. (i. e. the line becomes closer to the vertical line).