This function and the graphed function both have 4 as the amplitude. However, they have different periods. Perhaps you thought that $\displaystyle\frac{\pi}{{5}}\usepackage{cmbright}\Huge\$ was the period of the graphed function, but it is actually $\displaystyle\frac{{{2}\pi}}{{5}}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{y}={4}\cdot{\cos{{5}}}{x}\usepackage{cmbright}\Huge\$ .