Totally legitimate question.

If the target question is "Is this triangle isosceles?," then a statement will be sufficient if we can answer with EITHER yes OR no with absolute certainty.
That is, if we can say, with certainty, "yes, this triangle is isosceles," then the statement is sufficient.
Likewise, if we can say, with certainty, "no, this triangle is not isosceles," then the statement is sufficient.

Here's a different example:
Target question: Is x an even integer?
Statement 1: 2x + 1 = 13
When we solve the equation, we get x = 6, so we can say, with certainty, "yes, x is an even integer.
So, statement 1 is sufficient.

Another example:
Target question: Is x an even integer?
Statement 1: 2x + 1 = 11
When we solve the equation, we get x = 5, so we can say, with certainty, "no, x is not an even integer.
So, statement 1 is sufficient.

It's all about whether you can definitively answer the target question. It doesn't matter whether th actual answer is yes or no.

NOTE: Everyone struggles with Data Sufficiency (DS) questions at first. This question type is totally unique to the GMAT, so it's foreign territory.

If you're looking for some extra DS resources, we have a free set of videos that cover everything you need to know: https://www.gmatprepnow.com/module/gmat-data-sufficiency

Cheers,
Brent

Brent is absolutely right - if you can definitely answer "yes" or definitively answer "no," the statement is sufficient. If the result you get is "sometimes yes, sometimes no," then it's not sufficient.

That said, the vast majority of the time with DS questions, sufficient statements will give you a "yes" answer. You almost never have to worry about the definitive "no" case.

The "definitive no" case only becomes important on questions that you could rephrase in different ways. For example, DS Q39 asks "can the positive integer p be expressed as the product of two integers, each greater than 1?"

You could rephrase this question as "is p a non-prime?" You could also frame it the opposite way, "is p prime?" Because it's yes/no, these are effectively the same question.

Statement 1 says: 31 < p < 37

Since p can only be 32, 33, 34, 35, or 36, then p is definitely not prime. If you rephrased the question the first way, this would give you a "yes" answer, and if you rephrased it the second way, it would give you a definite "no" answer. But it still answered the question either way.

For more, see here: https://www.beatthegmat.com/ds-first-tim ... tml#653772