generating a randome Aritmetic operation

I am creating an android application which will generate a random question and Arithmetic operation I am able to create the question but I am having trouble to get the correct answer. For Example I can generate 3 + 3 *3 , But when I enter the answer which is 18 it always tell me that it is wrong. Any idea as how to get it right? I tried to use switch statement but it doesn't work. Any help will be much appreciated. Thanks