Slope fields and Euler’s method practice question 2

A drug is given by a steady intravenous drip and cleared by the body at a rate proportional to the amount present, so the amount $Q(t)$, in mg after $t$ hours, satisfies $\dfrac{dQ}{dt} = 8 - 0.4Q$. For exactly one value of the constant $b$, the function $Q(t) = b + Ce^{-0.4t}$ is a solution for every constant $C$. Find $b$.