The amplitude is correct, but the period is not. You may have thought that $\displaystyle{b}={4}\usepackage{cmbright}\Huge\$ , or you may have made a mistake dividing by a fraction. Note that $\displaystyle{5}{\cos{{\left(\frac{{x}}{{4}}\right)}}}={5}{\cos{{\left(\frac{{1}}{{4}}{x}\right)}}}\usepackage{cmbright}\Huge\$ , so $\displaystyle{b}=\frac{{1}}{{4}}\usepackage{cmbright}\Huge\$ . The amplitude of $\displaystyle{y}={a}{\cos{{b}}}{x}\usepackage{cmbright}\Huge\$ is given by $\displaystyle{\left|{a}\right|}\usepackage{cmbright}\Huge\$ , and the period equals $\displaystyle\frac{{{2}\pi}}{{\left|{b}\right|}}\usepackage{cmbright}\Huge\$ .