You may have used the number 8 cm as the value of s instead of the value of r. You need to use the formula $\displaystyle{s}={r}\theta\usepackage{cmbright}\Huge\$ for arc length with $\displaystyle{r}={8}\usepackage{cmbright}\Huge\$ cm, and $\displaystyle\theta=\frac{{3}}{{4}}\usepackage{cmbright}\Huge\$ .