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.
Hope that helps!!