Let $\displaystyle G = \left[\begin{array}{cc}a&a\\a&a\end{array}\right] | a \in R, a \ne 0$. Show that G is a group under matrix multiplication. Explain why each element of G has an inverse even though the matrices have 0 determinant.

Why isn't my latex code working? I want to produce a matrix with two rows, each rows with [ a a ]...