After finding $\displaystyle{r}=\frac{{10}}{{2}}={5}\usepackage{cmbright}\Huge\$ and using this to find the area of the circle, you probably used this number as the side length to find the area of the square. Then you subtracted in the wrong order. The side length of the square is 10. Find the area of the square and then subtract the area of the circle from this.