The cheetah must arrive at the same place at the same time:
x = 90t
650 + 60t = 90t
30t = 650
t = 650/30 ≅ 21.7 s
It takes the cheetah a little more than 20 s running at 90 ft/s to reach the moving antelope, so assuming the antelope can maintain its speed of 60, it is safe.
