 Solution

Use the left $A$-module structure $c\cdot (a\otimes b)=ca\otimes b$ on the tensor product of associative algebras. Expanding each $b\in B$ in the basis $b_1,\dots ,b_m$ shows that every tensor can be writtenThus the displayed family generates the module.

For uniqueness, let ${\beta }_j:B\to k$ be the corresponding dual basis, with ${\beta }_j(b_i)={\delta }_{ij}$. Applying the linear map ${\mathrm{id}}_A\otimes {\beta }_j$ to a linear relation ${\sum }_i{a}_i\otimes b_i=0$ gives $a_j=0$. Hence the family is linearly independent in a module, andIn particular $1\otimes b_1,\dots ,1\otimes b_m$ is a basis of a module, with coefficients uniquely in $A$.