You correctly found the amplitude and the orientation of this function, but the period is incorrect. Perhaps you recognized that the period of the graph is one fifth the period of $\displaystyle{y}={\cos{{x}}}\usepackage{cmbright}\Huge\$ , and thought that would make $\displaystyle{b}=\frac{{1}}{{5}}\usepackage{cmbright}\Huge\$ . But to find the value of b, you must set the period equal to $\displaystyle\frac{{{2}\pi}}{{\left|{b}\right|}}\usepackage{cmbright}\Huge\$ .