Explanation:

The middle three terms of this Arithmetic Progression represent the eleventh, twelth & thirteenth term.
The average of these three terms will represent the twelth term i.e. the 12th term will be 720/3 = 240.
The average of the last three terms will be the twenty second term i.e. 1320/3 = 440
The difference of twenty second term and the twelth term will give us ten times the difference i.e. 440 – 240 = 200/10 = 20 will be the difference.
If the twelth term is 240, the 18th term will be 240 + 6d i.e. 240 + 6 * 20 = 360 will be the 18th term.