In general, when we talk about a triangle and we've named the vertices with capital letters (your $\triangle ABC$ has vertices $A$, $B$, and $C$), we name the lengths of the sides opposite each vertex with the lower-case version of the letter labeling that vertex (the side opposite $B$ in your triangle is called $b$, which is the side with endpoints $A$ and $C$).
Since you've talked about $x$, $y$, and $r$ in other comments, I think you may be working with trigonometry on a coordinate plane. Let's try drawing a picture of the situation. We're talking about finding a trig function of angle $B$, so we want to put $B$ at the origin. We want the right angle, which is at $C$ to be on the $x$-axis. Since $a=5$ is the length of the side between $B$ and $C$, let's put $C$ at $5$ on the $x$-axis. With a right angle at $C$, point $A$ will be directly above or below $C$. Since $b=12$ is the length of the side between $A$ and $C$, let's put $A$ at $(5,12)$, directly above $C$.
Here's the picture so far:
Given this picture, do you know what the $x$, $y$, and $r$ are for finding the trig functions of angle $B$?