What is the differential rate expression of the change in b with time. incorporate elements that describe both its formation and reaction

The rate expression of the change in b with time is
rb = k [b]
This is for the formation of b assuming that the reaction is first order with respect to b
For the reaction equation, the expression is
-rb = -k [b]
Again, assuming that the reaction is first order with respect to b

Cr(oh)3 -> cr(3+) + 3oh-