Answer
a avg = 1.58 $m/s^2$
Work Step by Step
The magnitude of the acceleration is the magnitude of the acceleration vector. Since the vector was $ a = -(6/4)i + 0j + (2/4)k $ we can find the magnitude as $\sqrt (i^2 + j^2 +k ^2) $
This gives us $ \sqrt ((6/4)^2 + 0^2 + (2/4)^2) = \sqrt(2.25 + 0 + 0.25) = 1.58 $
