How Fast Were the Catapulted Boulders Initially?

[Casey314stl]

Homework Statement

A knight sits on a castle wall during a siege.
To while away the time, he notes that boulders catapulted from below land on the top of
his wall with a vertical velocity of 9.2 m/s.
If he is 40 m above the catapult, what
is the initial velocity of the boulders? The
acceleration of gravity is 9.8 m/s^2

Answer in units of m/s

Homework Equations

Δd=v^2f-v^2i-2a

The Attempt at a Solution

-10.4

 

[SammyS]

Homework Statement

A knight sits on a castle wall during a siege.
To while away the time, he notes that boulders catapulted from below land on the top of
his wall with a vertical velocity of 9.2 m/s.
If he is 40 m above the catapult, what
is the initial velocity of the boulders? The
acceleration of gravity is 9.8 m/s^2

Answer in units of m/s

Homework Equations

Δd=v^2f-v^2i-2a

The Attempt at a Solution

-10.4

What equation is that ?

v_f² = v_i² -2g(Δd)