A car starting From rest is accelerated to a velocity of 28m/s, a brake is apply in 4 second. Calculate its acceleration.
insufficient data.
All we know is that it will start slowing down when the brake is applied.
Now, if you meant that it accelerated for 4 seconds, then
a = 28m/s / 4s = 7 m/s^2
It's not my job to figure out what you mean.
It's your job to say it so clearly you cannot be misunderstood.