Answer:
686 days for sideral period of Mars.
615 days for synodic period of Venus.
Step-by-step explanation:
The equations we need to use is:
![(1)/(P)=(1)/(E)\pm(1)/(S)](https://img.qammunity.org/2020/formulas/physics/college/5sddlanpfjwyr25wspub02ebdfx77qfk9i.png)
Where P is the sidereal period, S the synodic period and E the Earth's period (365 days) and where the + is used for inferior planes (Venus) and the - for superior ones (Mars).
For the sidereal period of Mars we then have:
![P_M=(1)/((1)/(E)-(1)/(S_M))=(1)/((1)/(365days)-(1)/(780days))=686days](https://img.qammunity.org/2020/formulas/physics/college/82ifxr1hy5odi1i1e4w3g1jrsfrwqwnbez.png)
And for the synodic period of Venus we then have:
![S_V=(1)/((1)/(P_V)-(1)/(E))=(1)/((1)/(229days)-(1)/(365days))=615days](https://img.qammunity.org/2020/formulas/physics/college/t7yi5jju09ryrdufl7kbyq0afr82sujibg.png)
(Commonly for Venus a sidereal period of 225 days is given, which changes this result to 587 days).