The sum of an infinite geometric series can be found using the formula:
S = a / (1 - r)
where "a" is the first term of the series and "r" is the common ratio between consecutive terms.
In this case, the first term is 3 and the common ratio is -1/3. So we can plug these values into the formula:
S = 3 / (1 - (-1/3))
S = 3 / (4/3)
S = 9/4
So the sum of the infinite series is 9/4.