I have been trying to solve this for hours but without success. I tried to write down the definitions which means that our function can also be written as
$$u= \frac{\sum_{j=1}^{n} |x_{j}|^{2}}{\sqrt{ \left(\sum_{j=1}^{n} |x_{j}|^{2} \right)^{n}}}$$
and then we have defined $$\Delta u = \nabla \cdot (\nabla u) = \sum_{i=1}^{n} \frac{\partial ^{2}\|x\|_{2}^{2-n}}{\partial x{_{i}}^{2}}$$
Now here I am totally stuck. If somebody could help me I would be very thankful