Average acceleration?

Question

An aircraft has a takeoff velocity of 180km/h. What length of runway is needed. if the aircraft average acceleration along the ground is 2.5m/s

Answer 1

x = v²/2a = (180/3.6)²/(2*2.5) = 500 m

Answer 2

t = 50 m/s / 2.5 m/s^2 = 20 s

d = (0 m/s + 50 m/s)/2 * 20 s = 25 * 20 m = 500 m

I believe that's right, my physics is rusty though. I know it can be done with one formula, but I can't remember it exactly off the top of my head, so I just figured it separately.