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There are two ways to solve this problem.
One way is Algebra. You need to use this equation for the height h(t) = -16*t^2 +70t + 3; find the time moment when h(t) = 5 ft
(on the way up or down - it does not matter !); then calculate the velocity using the given formula for that value of "t".
This way is long and boring.
Another way is to use the energy conservation law of Physics, which says
= - , (1)
where v = 70 ft/s is the given velocity; V is the velocity under the question at the height of 5 ft;
delta_H = 2 ft is the difference of levels; m is the mass of the ball and g = 32 ft/s^2 is the gravity acceleration.
Cancel "m" in both sides of the equation (1) to get
= - , (1)
substitute the values
= - .
and complete the calculations on your own. You goal is to find the capital V !