Question

what is the initial speed of a body projected vertically upward if it takes 8sec to come to its original position?

Answer 1

Gravity has a constant pull downwards, technically known as acceleration. Distance is not important as we don't need to know how high it went, we know the initial and final velocities will identical as it will loose speed on the way up as it gains it back on the way down and we have the total flight time.

In all, we have v=u, t=8 sec and a =9.81m.s^-2 (m/s^2).

I'll leave you to find the formula with these variables.