Many Thanks in advance, I dont know where to start on this one
Which of the following sets are groups with respect to matrix multiplication
a) real 2 × 2 matrices with positive determinant.
b) invertible real 2 × 2 matrices of the form
/ a b \
\ c d /
In each of the cases below decide whether the setS is a subgroup of the group G.c) G = GL2(R) and S the set of all matrices in G of the form / a b \
\ -b a /
d)G the group S8 and S the set of all permutations in S8 that fix at least 3 numbers.