$${u_m = \vert -2 (2,00 \times 10^{-3}\, \mathrm{m})(2 \pi)(806,2\, \mathrm{Hz}) \\ \times \left( \frac{2 \pi}{0,400\, \mathrm{m}} (0,180\, \mathrm{m}) \right) \vert \\ = \underline{\underline{6,26\, \mathrm{m/s}}}}$$