Question

Find f'(a). f(t) = 2t^3 + t

Answer 1

$f(t)=2t^3+t\\\\f'(t)=3\cdot2t^{3-1}+1=6t^2+1\\\\\boxed{f'(a)=6a^2+1}$

Answer 2

Using derivations laws 
f(t) = 2t^3 + t
f'(t)=6t^2+1
substituting a instead of t
f'(a)=6a^2+1
hope it helps