I will set this up for you and you can take it from there.
You will have to find the velocity v from r, find speed v from the components of v, and then minimize v.
v = dr/dt = 2ti + 9j + (2t - 16)k
v = √[(2t)2 + 92 + (2t - 16)2]
At a maximum or minimum,
dv/dt = 0
Find dv/dt, set it equal to 0, and solve for t. You will have to perform a test to see if the value(s) of t give(s) a maximum or a minimum.