You may have been thinking about the function $\displaystyle{f{{\left({x}\right)}}}={3}{x}\usepackage{cmbright}\Huge\$ , for which the domain and range are all real numbers. However, the domain is restricted because you must have $\displaystyle{3}{x}\ge{0}\usepackage{cmbright}\Huge\$ . So the domain is $\displaystyle{x}\ge{0}\usepackage{cmbright}\Huge\$ . Because the square root must always be positive or 0, the range is $\displaystyle{f{{\left({x}\right)}}}\ge{0}\usepackage{cmbright}\Huge\$ .