This graph has the correct amplitude and period and it does look like a cosine graph with a hill occuring on the y-axis. However, you should also pay attention to the negative sign in $\displaystyle{y}=-{3}{\cos{{\left(\frac{{1}}{{2}}{x}\right)}}}\usepackage{cmbright}\Huge\$ . This will cause the graph to be a reflection of the graph of $\displaystyle{y}={3}{\cos{{\left(\frac{{1}}{{2}}{x}\right)}}}\usepackage{cmbright}\Huge\$ over the x-axis, making it so that there will be a valley occuring on the y-axis.