Hello aspirant,

I guess your average speed is calculated  the change in position to change in time. So here the change in position should be calculated by difference in position at t2 and t1 and change in time interval is (t2-t1).

OR

You could directly differentiate f(t) and get average speed as 2at + b.

The final equation should look like a(t2+t1) + b.

You basically need to find the point in time at which it attains an average speed of 2at + b.

The point at which the above equation is equal to 2at +b will be when you substitute t=t2+t1/2.

I hope this information helps!

Thank you!