Answer
0.89 Pa/cm.
Work Step by Step
Use Poiseuille’s equation with a length of 1 cm. The volume flow rate is the area of the aorta multiplied by the speed of the moving blood.
$$Q=\frac{\pi R^4 (P_1-P_2)}{8 \eta \mathcal{l}}$$
$$ \frac{ P_1-P_2}{l}=\frac{8 \eta Q}{\pi R^4}=\frac{8 \eta \pi R^2 v}{\pi R^4}$$
$$ \frac{ P_1-P_2}{l}=\frac{8 (4\times10^{-3}\;Pa \cdot s) (0.40\;m/s)}{ (0.012\;m)^2}=88.9\;Pa/m=0.89\;Pa/cm$$