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 half the period of $\displaystyle{y}={\cos{{x}}}\usepackage{cmbright}\Huge\$ , and thought that would make $\displaystyle{b}=\frac{{1}}{{2}}\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\$ .