The following image explains where the formula $R=2P+S$ comes from.

Figure: Planetary gear with 4 planets (https://woodgears.ca/gear/planetary.html)
Although it has 4 planets the spacing will be the same with 2 or more planets. (Three is used for optimal balance and redundancy).
Essentially, the diameter of the Ring $d_R$ should be equal to the Diameter of the sun $d_S$ plus 2 diameters of the planets $d_p$. I.e. :
$$d_R = 2d_P+ d_S \tag{eq:1}$$
However for gears, the diameter of a gear ($d$) and the number of teeth ($z$) are proportional.
- in the metric system usually the module m is used, $d= m\cdot z$
- in the US system usually the Diametral Pitch (P) is used, $d= \frac{z}{P}$
In both cases, the equation 1 becomes:
$$z_R = 2z_P + z_S$$
So for $z_R=75$ and $z_S=5$, the correct number for the teeth of sun is $z_S = 75-2*5=65$