Find the image of the point (2, 1) when it is reflected in the line $x+y=7$.

The image (a, b) must make the join perpendicular to the line, so $a-2=b-1$, and the midpoint must lie on the line: $\frac{a+2}{2}+\frac{b+1}{2}=7$. These give $a=6$ and $b=5$. (4, 3) is only the midpoint.