Question

A ball is thrown directly downward with an initial speed of 8.75 m/s, from a height of 21.0 m. after what interval does the ball strike the ground

Answer 1

there you go!
as it is unnecessary to find velocity, so use the equation with v
remember to take all directions downwards as it is more convenient to calculate
after substituting the values, you have to reject the negative answed as time must be+
:)