## Thomas' Calculus 13th Edition

$50$ seconds
Since $$x=\left(v_{0} \cos \alpha\right) t$$ Then $$(21 \mathrm{km})\left(\frac{1000 \mathrm{m}}{1 \mathrm{km}}\right)=(840 \mathrm{m} / \mathrm{s})\left(\cos 60^{\circ}\right) t$$ Hence $t=\frac{21,000 \mathrm{m}}{(840 \mathrm{m} / \mathrm{s})\left(\cos 60^{\circ}\right)}=50$ seconds