You may have correctly drawn the angle in standard position, starting on the x-axis, moving counter-clockwise, and ending in the fourth quadrant. However, a reference angle will have a value anywhere from $\displaystyle{{0}}^{\circ}\usepackage{cmbright}\Huge\$ to $\displaystyle{{90}}^{\circ}\usepackage{cmbright}\Huge\$ . It will never be negative.