-a would be because as you can see there is a pattern of sticks, so what i did was count up the sticks and i found that there was 6 in the first one. So i counted the second one and it was 10 so i realized that all i had to do was add 4.
and for b i think when it says n it means the nth term which is 4 so i think it means pattern 4 which would be 18 so try that out
