337. Find the condition under which the numbers $a, b, c$ would be the $p$-th, $n$-th, $k$-th terms of the same arithmetic progression.
Instruction. For the previous task, the required condition will be:

$$
\frac{b-a}{c-b}=\frac{n-p}{k-n}
$$

or

$$
a(n-k)+b(k-p)+c(p-n)=0
$$