Hi Olivia!
Given that the horse has a constant acceleration, we can use the kinematic equation:
V2 = V02 + 2·a·Δx
where V is the velocity after the horse has traveled a distance of Δx at a constant velocity of a starting from an initial speed, V0. The goal is to solve for the acceleration, so we can rearrange the kinematic equation by first subtracting V02 to the left side:
V2 - V02 = 2·a·Δx
Then we can divide both sides by 2·Δx:
(V2 - V02)/(2·Δx) = a
Based on the problem, the horse starts at rest, so V0 = 0. The horse travels 100 m, so Δx = 100 m, and the horse reaches a speed of 12 m/s, so V = 12 m/s. Plugging in these values into the equation above:
a = ((12 m/s)2 - 02)/(2·(100 m)) = (144 m2/s2)/(200 m) = 0.72 m/s2
I hope this helps!
Seth