This graph has the general shape of a cosine function, so $\displaystyle{y}={a}{\cos{{b}}}{x}\usepackage{cmbright}\Huge\$ . The amplitude is 1. The graph of $\displaystyle{y}={\cos{{x}}}\usepackage{cmbright}\Huge\$ has a hill centered around the y-axis, while this graph has a valley centered around the y-axis. So there has been a reflection over the x-axis and this means that $\displaystyle{a}=-{1}\usepackage{cmbright}\Huge\$ .

Now there is a full cycle that runs from $\displaystyle-\frac{\pi}{{4}}\usepackage{cmbright}\Huge\$ to $\displaystyle\frac{{{3}\pi}}{{4}}\usepackage{cmbright}\Huge\$ , so the period is $\displaystyle\pi\usepackage{cmbright}\Huge\$ . Since $\displaystyle\pi=\frac{{{2}\pi}}{{\left|{b}\right|}}\usepackage{cmbright}\Huge\$ , the value of b could be 2. So this could be the graph of $\displaystyle{y}=-{\cos{{2}}}{x}\usepackage{cmbright}\Huge\$ .