If the shear stress on section $\mathrm{a}-\mathrm{a}$ is $25 \mathrm{MPa}$, then the shear force component $\mathrm{V}$ can be computed as:

$$\begin{gathered} \mathrm{\tau }=\frac{\mathrm{V}}{{\mathrm{A}}_{\mathrm{a}-\mathrm{a}}} \therefore \mathrm{V}=\mathrm{\tau } {\mathrm{A}}_{\mathrm{a}-\mathrm{a}} \\ \mathrm{V}=(25 \mathrm{N}/{\mathrm{mm}}^2)(850.65 {\mathrm{mm}}^2) \\ =21,266.25 \mathrm{N} \\ =21.266 \mathrm{kN} \end{gathered}$$