jmc

We can't read off the exact value of $u(-2.33)$ or $u(-0.81)$ or $u(0.81)$ or $u(2.33)$ from the graph. However, the symmetry of the graph (under $180^{\circ }$ rotation around the origin) tells us that $u(-x)=-u(x)$ for all $x$ in the visible interval, so, in particular, $$u(-2.33)+u(2.33)=0$$and $$u(-0.81)+u(0.81)=0.$$Thus, the exact value of $u(-2.33)+u(-0.81)+u(0.81)+u(2.33)$ is $\boxed{0}$.