What do all linear functions of the form $f(x) = ax + a$, $a > 0$, have in common?

Here, $f(x) = ax + a = a(x + 1)$.

So, $f(x) = 0$ when $x = -1$.

All these lines pass through the same point $(-1, 0)$.

Therefore, regardless of the value of $a$, the $x$-intercept is always $-1$.