A rocket moves upward for 3 seconds
After 2 seconds, its speed is 10 m/s
After another 1 second, its speed is 20 m/s
Find instantaneous acceleration at t=2.5
More information is needed.
The rocket does not make uniform acceleration because the velocity increases 10 m/s in the first two seconds, and another 10 m/s in the next ONE second.
So we have three points for a curve, but the curve is not defined. We know that it is continuous, but it could be two straight segments, a parabola, or any other shape.