I️’m assuming you mean he starts at 0 m/s. In that case, v = 12, u = 0, and t = 4, meaning that 12 = 0 + a*4, or that a = 3 m/s^2 for the first 4 seconds. The fact that the total distance is 200 m seems irrelevant since it doesn’t tell you that he runs the whole thing in that 4 seconds (he doesn’t).
