mam, not getting what you are saying. $G/N$ is a set of left/right cosets and it also have a group properties. Considering right cosets, $G/N = \{Na_1,Na_2,....\}$, where $a_1,a_2,....$ are the elements of the group G which are matrices here and in every matrix, a,b are real numbers. So, a,b can be any real no. $Na_1,Na_2,...$ are the right cosets where N is a set of matrices of type $\begin{bmatrix} 1 &b \\ 0&1 \end{bmatrix}$ here b can take any real value.