$\text{Rank} \begin{pmatrix} A & X \\ 0 & B \end{pmatrix} \geq \text{Rank}(A) + \text{Rank}(B)$ for any $n \times n$ matrices $A,B, X$

Question

Question: Let $A,B$ and $X$ be $n\times n$ matrices, I need to show that

\begin{align} \text{Rank} \begin{pmatrix} A & X \\ 0 & B \end{pmatrix} \geq \text{Rank}(A) + \text{Rank}(B). \end{align}

This is exercise 26, page 16 in the book Matrix theory by Zhang. I was trying to review linear algebra again and got stuck at this problem. Any hint would be highly appreciated. Thank you!