It should be ... with

... because1. if , then

Let x=[a b : 0 1] and y =[c d : 0 1]

Then

xy=[ac ad+b : 0 1]

... i.e. the identity element of belongs to .2. If a=1, b=0, then I = [1 0 : 0 1]

... i.e.3. if , then .

x=[a b : 0 1], then x^{-1} = [1 -b : 0 a]

thus, .

P.S. If you use the characterization theorem of subgroups you only need to prove and for all .