Here's a formula for the distance an object falls from rest:
D = (1/2) A T² ... <== MEMORIZE THIS
Distance = (1/2) · (acceleration) · (time²)
On Earth, the acceleration of gravity is always 9.8 m/s². So for a fall that starts with zero speed, the formula says
70 m = (1/2) (9.8 m/s²) (time²)
70 m = (4.9 m/s²) (time²)
Divide each side by (4.9 m/s²) ;
time² = (70 m) / (4.9 m/s²)
time² = (70/4.9) sec²
time = √(70/4.9) seconds
time = 3.78 seconds