Hello,

We know that, as per the Equation of Motion,

s = ut + 0.5 a t^2

So, for first 3 sec.,

10 = 3u + 0.5 x a x 9

So, 3u + 4.5a = 10.............. ( 1 )

Now, for next 4 sec.,

25 = 7u + 0.5 x a x 49

So, 7u + 24.5a = 25............ ( 2 )

Solving ( 1 ) and ( 2 ), we get,

a = 0.12 m/s^2 and u = 3.15 m/s

Now, also as per the Equation of motion,

v = u + at

So, For 8 sec.,

v = ( 3.15 ) + ( 0.12 x 8 )

So, v = 4.11 m/s

Hence, The velocity of object at the end of 8 seconds will be 4.11 m/s

Best Wishes.