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:

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:

And for the synodic period of Venus we then have:

(Commonly for Venus a sidereal period of 225 days is given, which changes this result to 587 days).