Let $A$ be an $m$x$m$ nonnegative definite matrix, while $B$ is an $n$x$m$ matrix. Show that $BAB'$ is a nonnegative definite matrix.
I know by definition that a nonnegative definite matrix has the following definition: $x'Ax \ge 0$. However, I do not know how to employ this definition to arrive to the answer.