Hi, I have a kinematics question which i can't solve and i need help. Here it is.

A particle moves in a straight line passing a fixed point O with a velocity of 3m/s. It moves in such a manner that t seconds after passing O, its velocity is given by v= atē+b. If the particle is again at O after 3 seconds, find its speed at that instant. Find the total distance traveled between t=0 and t=3.

This is what i have tried so far :

v= atē+b

subst. t=0, v=3.

3 = a(0)ē + b

b=3

∴ subst. b=3, t=3 into v= atē+b

v = a(3)ē + 3

v = 9a + 3

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I couldn't continue after arriving here, can't get the unknown,a. I only need help here. I understand how to get the answer for the distance part. Thanks