Ultrasound is often used to make images of developing fetuses. In particular, by measuring the size of a fetus ultrasound technicians can estimate its age and then predict its birthdate. To do this requires formulas for fetus size as a function of age. Verbung et al. (2008) fit data from over 6000 fetal ultrasounds. They measured the femur length, L, (in mm) as a function of the fetus age, t, (in weeks) and found the following formula

L= -37.50 + 3.71t- 6.33 x 10^-4t^3.

Required:
a. Calculate the rate of growth, dL/dt, at t =15, 20, and 30 weeks.
b. Write a sentence interpreting L'(15) value in terms of the femur length.