Linear Transformations · Medium problem

Let $A = \begin{pmatrix}1 & 2\\0 & 1\end{pmatrix}$ and $B = \begin{pmatrix}1 & t\\0 & 1\end{pmatrix}$, both shears. For which value of $t$ does $B$ exactly undo $A$, i.e. $AB = I$?