1. The problem asks to find the output of the function $f$ at $x=3$, i.e., find $f(3)$. 2. The graph shows a straight line passing through points $(-6,-6)$, $(0,0)$, and $(6,6)$. 3. From these points, we see the function is linear and can be described by the formula $f(x) = x$. This means the output equals the input. 4. To find $f(3)$, substitute $x=3$ into the function: $$f(3) = 3.$$ 5. Therefore, the output of the function at $x=3$ is 3.