There are quite a few questions here. I believe the moderation team likes members to limit the number of questions to around 3 per post. However, here is assistance with the first four:

1.S=3/1-r if r= -3/4 , S=?

By subbing in the value of r:

Can you solve it from there?

2.

Actually, this question makes no sense. No n value exists in the original equation, so you're unable to substitute n=8 anywhere.

- I will assume the -1 is not part of the exponent.

3.

Can you take it from there?

4.write the quadratic equation whose roots are 2/3 and -1

By using null factor rule:

Therefore,

Can you solve it from there?