Hi,
when I read the question and saw the image, I assumed that angle ACB is right angle because it was not written that "figure not drawn to scale". What should I do in the GMAT axam? can I assume right angle? Why the comment "figure not drawn to scale" is not there?
and one more question- how can I insert the image?

A few general points (they'll have to be general, since I don't know what question you're talking about):

-When a question is not marked 'not drawn to scale' the solution will match the figure.

-That does not mean that you can always use apparent relationships in the figure to answer a geometry question!

-For DS questions, the actual values are (just about) irrelevant. You need to answer how much info is sufficient to determine, say, the area of a rectangle. You do not need to calculate the area.

-For PS questions whose answers are close together, you cannot much rely on the figures. You can tell the difference between a 90 degree angle and a 88 degree angle.

-Some features of figures are reliable even when the figure is not to scale. Straight lines look like straight lines, vertices look like vertices, etc.

-Having said all that, you can often productively estimate in PS geometry questions.

-Even in DS questions the apparent proportions can give you hints. Does that chord AB appear to be a diameter? If making it a diameter would allow you to answer the question, then rephrase the question as 'Is AB a diameter?', etc.