The concentration of mRNA in a crop overtime is modelled by the function f(t)=12e−t(sin(t)−cos(t))+ 1/2 . Determine the rate of change of mRNA concentration as a function of time?

If you meant f(t)=12e^(−t(sin(t)−cos(t)))+ 1/2

that would be
df/dt = 12e^(−t(sin(t)−cos(t))) * ((sint-cost) + t(cost+sint))