How many different solutions does the following inequality have, in which $x_1$ and $x_2$ must be non-negative integers and $x_3$ must be a positive integer?
$x_1 + x_2 + x_3 \leq 13$
Struggling with this question. I know how to do it if all $x$'s are non-negative but the one positive $x$ is throwing me off.