Find the CFG for $a^n b^m c^k$

Find the CFG for $a^n b^m c^k$

I have a language like this and will need to write the Context-free
grammar for it.
$$\{a^n b^m c^k\; : \;k = |n-m|,\; m, n, k \geq 0\}$$
What I am confused is do I need to simplify the condition of k = |n-m| to
solve or it just means the length