\(\frac{{({n_2} - {n_1})\left[ {({n_2} - {n_1}) + 1} \right]}}{2}\,\, = \,\,\frac{{\left( {6 - 3\left[ {\left( {6 - 3} \right) + 1} \right]} \right)}}{2}\,\, = \,\,\frac{{3 \times 4}}{2}\,\, = \,\,6\)