If two fair dice are rolled at the same time, what is the probability that of it obtaining two multiples of 3?

How many multiples of three are there in a dice?
6 and 3. So 2.
And there are 6 possible outcomes for each dice, and both dices added there are 12 possible outcomes. This number will be your denominator. 
So since there are 2 in each dice, add them to equal 4, and put the 4 as the nominator. 
Anser: 4/12.

Im assuming this is a non biased 6 sided dice,

multiples of 3 would be 3 and 6 therefore chance of rolling one would be 2/6 = 1/3.

however since the question says two dice at the same time, you have to multiply the probability which would be 1/3 x 1/3 = 1/9