The graph of a sine function would have a hill or valley on each side of the y-axis, while a cosine function would have a hill or valley with the y-axis running through the middle of it. This graph has a hill to the left of the y-axis, so it has the form $\displaystyle{y}={a}{\sin{{b}}}{x}\usepackage{cmbright}\Huge\$ .

The graph of $\displaystyle{y}={\sin{{x}}}\usepackage{cmbright}\Huge\$ has a valley to the left of the y-axis. Because the graph in question has a hill there, a reflection has taken place. This implies that $\displaystyle{a}<{0}\usepackage{cmbright}\Huge\$ .