@6: being an arithmetic sequence with the 5th term 10 and the 7th term 18

the difference between two terms is 4 (the difference between 3 terms being 8)
and therefor the 8th term is 18+4=22

@7:

a{2}=a{1}-a{0}=3-0=3
a{3}=a{2}-a{1}=3-3=0
a{4}=a{3}-a{2}=0-3=-3

@9: write 17+21+25+...+417 as summation

the difference between the terms is 4 and the number of terms is 101
there are various slightly different ways to write it as summation

one, probably the simplest, is sum[i=0,100](17+4*i]

#12 idk any special tricks, so you could just multiply it out. It looks like it take 3-4 iterations

(2a-b)^8

(2a-b)*(2a-b) = 4a^2-2ab-2ab+b^2
then you multiply more
(2a-b)*(4a^2-4ab+b^2) = etc. etc. until you get a^3b^5 and the coefficient is the number in front of it

Doubt you still need help but i'm a math major and would love to help ya out if needed!