Lol! Nothing to be embarrassed about, it's not the kind of math most people do every day.
How much algebra\geometry do you know? Is there something specific I could walk you through, or is it all soup?
Yes, you could do it by "guess and check". Or, better yet, just enter 1 degree--or less than a degree if you need more precision--in the angle for the Rotate button. Keep hitting the button until your shape looks aligned. Count the number of times you hit the button, and that's the angle you rotate by. Again, you'll need to do each axis one at a time.
Make sure you perform your rotations in the same order that the game is expecting. Most games want rotations in X,Y,Z order, but occassionally they might ask for something else. Also, make sure you enter your angle in degrees or radians as the game expects. You can use google to convert
(ie. Search 'convert 90 degrees to radians', etc.)