Possible values of a parameter in an arithmetic progression

Question

In the following question .

How do they get to know that the possible values of $a$ are only $3$ and $4$?

Answer 1

$\lambda$ has to be an integer, so $a$ cannot be $2$. It has to be positive, so $a$ cannot be $1$ or greater than $5$, and it has to be finite, so $a$ cannot be $5$. Finally, $a$ itself must be positive. Thus $a$ can only be $3$ or $4$.

Answer 2

Well, we know that a, b, c, and d are positive integers. If we try a=4 or anything greater than 4, we get the common difference to be negative, which cannot be so since we are given the arithmetic progression a