two problems i have differentiated. need to find stationary point and state if max or min value.

$\displaystyle (a)y= (3x+1)^4$

$\displaystyle dy/dx= 12(3x+1)^3$

this is where im stuck. do i make dy/dx= 0?

$\displaystyle (b)x=\frac{1}{4t^2+3}$

$\displaystyle dx/dt = -8t(4t^2+3)$

many thanks