what is CFG of $\{a^i b^j c^k \mid k = |i-j| \}$
Use the fact that CF are closed under union, and start by considering cases, whether $i\ge j$ or $j\ge i$.

In the first case we get the language $L_1 = \{ a^i b^j c^k \mid k+j=i\}$, which is the same as $L_1 = \{ a^{k+j} b^j c^k \mid k,j \ge 0\}$.

Make a grammar for $L_1$, and then consider the second case.