I am assuming this is a trapezoid.
The area of a trapezoid is : A=(B1+B2)H/2
Plug in the numbers to get:
18=(6+B2)*2/2
18=6+B2
18-6=6-6+B2
B2=12
Answer:
The next number in the sequence is: 13
Step-by-step explanation:
So we start with 3, then the pattern is:
subtract 9, add 18, subtract 8, add 16, subtract 7, add 14...
If we apply this to the pattern we can see that it works. You should be able to continue the pattern if needed.