Word Problem with Kinematics Equation

[Natko]

Homework Statement

An object thrown up from a cliff 10 m/s reaches a velocity of 20 m/s [down] as it lands. If acceleration due to gravity is 9.8 m/s², what is the object's displacement? How long did it take for the object to land from the time it was thrown?

Homework Equations

I'm not sure if all of the below are relevant, or perhaps some equations are missing.

Δx = v₀t + 1/2at²
(displacement = initial velocity x time + 1/2 acceleration x time squared)

v_f² = v₀² + 2aΔx
(final velocity squared = initial velocity squared + 2 acceleration x displacement

The Attempt at a Solution

t₁ = v/a = 1.02 s
t₂ = v/a = 2.04 s
t₁ + t₂ = 3.1 s

Not sure how to find the displacement, but the answer key says it's -15m. How do I find it?

 

[Doc Al]

Not sure how to find the displacement, but the answer key says it's -15m. How do I find it?

You can use either of your two equations to solve for the displacement. If you use the first, do not round off the time to 3.1 seconds.