The car travels a distance d from rest with acceleration a after time t of
d = 1/2 a t²
It covers 69 m with 2.8 m/s² acceleration, so that
69 m = 1/2 (2.8 m/s²) t²
t² = 2 (69 m) / (2.8 m/s²)
t ≈ 7.02 s
where we take the positive square root because we're talking about time *after* the car begins accelerating.