For a. they want you take the derivative of that function but in terms of t, which means you have to plug in that x=... part at some point.
You can either plug it in first and then take a derivative or take the derivative first then plug it in. But remember if you take a derivative first then u'=2x³(x') due to the chain rule; so you'd need to plug in the derivative of x anyways. It's really just a matter of preference at that point.
Doing it either way gives us:
u'=2(2t³-21t)³(6t-21)
For b. you just need to plug in 2 and simplify to get your answer there.
