If $A$ and $B$ are two matrices of order $3\times m$ and $3\times n$ respectively and $m=n$, then the order of $5A-2B$ is

If $A$ is a matrix of order $m\times n$ and $B$ is a matrix such that $A{B}^T$and $B^{T}A$ are both defined, then the order of matrix $B$ is