Answer
a = -18i $m/s^2$
Work Step by Step
The velocity is given by $ v = (6t-4t^2)i + 8j $
The acceleration is given by the derivative of the velocity so we need to find:
$\frac{d}{dt}((6t-4t^2)i + 8j)$
We find this to be
$a = (6 -4(2)t)i + 0j = (6-8t)i $
To find the acceleration at t = 3 we just need to plug in 3 for t in the expression above which gives
$a = (6-8(3))i = (6-24)i = -18i $
