The correct answer is 30. You must follow the pattern that is given.
The width as you can see, goes up by 3 every step. The length goes up by 5 every step. So, if the calculations are correct and the pattern is followed:
Width: 3 6 9 12 15 18
Length: 5 10 15 20 25 30
The 6th term of the width is 18 so the 6th term for the length is 30.