first.... well done for scoring 50....

now for the questions:
question 1:
there are 4 possible "session attendance" outcomes:
A: no session
B: morning only
C: afternoon only
D: morning and afternoon

each person of the 128 is exactly in one group.
stem tells us that group A is empty. great.
it also asks how many people are in group B....

st1 tells us that group D has 3/4 of the people.... which means the rest are divided between B and C... so is insufficient.
st2 tells us that "7/8 of the people attended the afternoon session"... what does it mean? which groups from A,B,C,D attended the afternoon session? oh... that's C and D. so C and D together holds 7/8 of the people. where can be the remaining 1/8? true... in group B, which is exactly what we wanted to know. so st2 is sufficient.

answer is B.


question number 2:
to understand your mistake just plug-in x=2 and x=-2 and see that the answer is not always 1. this same example will show you that the result is neither -1,0 nor x. so by elimination you can safely deduce that the answer is E.

question number 3:
lets denote angle QRS by a, RSQ by b, STU by c and TSU by d.
we know that a+d=90 (because it is right triangle).
we also know that c+b+x=180 or x=180-(b+c)
so we miss lots of info.
st1 and st2 are clearly insufficient alone (each one deals with one side and provide no info on the other side of the problem).
lets check them together..
from st1 we know that QRS is isosceles, which means b=(180-a)/2
from st2 we similarly infer that c=(180-d)/2
b+c = (360-a-d)/2 = 270/2 = 135 (because a+d = 90)
so x=45
hence st1,st2 together are sufficient.