Chapter 5, Problem 72P

Three forces act on a particle that moves with unchanging velocity $\stackrel{\to }{v}=\left(2\text{ m/s}\right)\hat{i} -\left(7\text{ m/s}\right)\hat{j}$. Two of the forces are ${\stackrel{\to }{F}}_1=\left(2\text{ N}\right)\hat{i} +\left(3\text{ N}\right)\hat{j} +\left(-2\text{ N}\right)\hat{k}$ and ${\stackrel{\to }{F}}_2=\left(-5\text{ N}\right)\hat{i} +\left(8\text{ N}\right)\hat{j} +\left(-2\text{ N}\right)\hat{k}$. What is the third force?