Infact brother suleiman is right. A square triangle is a logical impossibility and hence it can not exist so the question whether Allah can create a "square triangle" or not is superfluous and meaningless. Even if a person responds to such a question with a NO , it does not effects Allah 's omnipotence as the question is referring to a logical impossibility which can not exist/ Same logical impossibilities are posed in various other questions as well , like "Can God make a stone which he can not lift himself?" , "Can God lie?" etc etc.