The question is asking for the highest common factor. For a common factor, look at the number's prime numbers.

Take out the common ones, so in this case, 2 and 3. Ignore 5 and 11. From there, between the two numbers, choose the one with a lower power.

So 792= 2^3 × ...
While 990= 2 x ...

So in this case, take 2, instead of 2^3, because 990 does not have the factor for 2 to be more than 2.

This question is special because 3 has for both numbers have the same power. So just take 3^2

I hope this helps :)