AQA GCSE Physics (9-1): Uniform Acceleration.

Answer:
a=-4m/s2 (negative for deceleration)
u= 23m/s
s = 56m

so:
v^2 - (23)^2 = 2 x -4 x 56
v^2 = 23^2 + (-448)
v^2 = 81
v = sqrt(81)
v = 9m/s