Linear algebra.Kernel of a linear mapping

[manuel325]

Let $L$ be a linear operator ::$L(A)= Tr(A)$ where $Tr(A)$ is the trace of a square matrix
Find a basis of the kernel of L.
Any help would be really appreciated . thanks in advance

 

[CompuChip]

How about we start with the definition?
If A is an nxn matrix, what does it mean for A to be in ker L?