Answer
$v_f=26.5m/s$
Work Step by Step
Using the conservation of energy formula;
$$K_i+U_i=K_f+U_f$$ $$\frac{1}{2}m{v_i}^2+mgh_i=\frac{1}{2}m{v_f}^2+mgh_f$$ $$\frac{1}{2}{v_i}^2+gh_i=\frac{1}{2}{v_f}^2+gh_f$$ $$\frac{1}{2}{(17 m/s)}^2+(9.81 m/s^2)(42.0m)=\frac{1}{2}{v_f}^2+(9.80m/s^2)(21.0 m)$$ $$v_f=26.5m/s$$
