Since acceleration is uniform, if the velocity at point B is v, then
[tex]v^2 - \left(20\dfrac{\rm m}{\rm s}\right)^2 = 2\left(3\dfrac{\rm m}{\mathrm s^2}\right)(120\,\mathrm m)[/tex]
Solve for v :
[tex]v^2 = \left(20\dfrac{\rm m}{\rm s}\right)^2 + 2\left(3\dfrac{\rm m}{\mathrm s^2}\right)(120\,\mathrm m) \\\\ v = \sqrt{\left(20\dfrac{\rm m}{\rm s}\right)^2 + 2\left(3\dfrac{\rm m}{\mathrm s^2}\right)(120\,\mathrm m)} \\\\ \boxed{v \approx 34 \dfrac{\rm m}{\rm s}}[/tex]
