Could anyone help me please? I don't have any examples of how to work out the centre of a group, so I'm struggling. I keep trying to apply the definition but any way I try doesn't seem to work.

Work out the centre of

$\displaystyle T(2,R)=

\{ \left(\begin{array}{cc}a&b\\0&c\end{array}\right); a,b,c $ elements of $\displaystyle R, ac \neq 0 \}

$

The $\displaystyle R$ should be the symbol representing the real numbers but I don't know how to put that in.

Thanks.