First, from the formula given we solve the equation for D:

Now, substituting the given data:
(a) PD=405 cu in, L=4.7 in, and N=7,
in the above equation we get:
![D=\sqrt[]{(4*405)/(4.7*7*\pi)}in=\sqrt[]{15.6736175}in\approx3.96\text{ in}](https://img.qammunity.org/2023/formulas/mathematics/college/40taj1wdtc2m7wtcsixte23unb0myn3kkf.png)
(b) PD=399.4 cu in, L=2 in, and N=6,
in the above equation we get:
![D=\sqrt[]{(4*399.4)/(2*6*\pi)}in=\sqrt[]{42.37765618}in\approx6.51\text{ in}](https://img.qammunity.org/2023/formulas/mathematics/college/k92clk2i7mujeq2w33vqsienlgy3ynmb1x.png)
Answer:
(a) 3.96 in.
(b) 6.51 in.