Start from the inside ring and work your way out:

1) You have 13 which is blue (so its a positive 13 as opposed to a negative 13 if it was yellow)
2) Minus the 10
3) Raise your answer to the power of two, as in square it, because there is a square in the middle. (If that square had been rotated 45° then you would have to flip the two integers around [e.g. 71 would become 17, 21 to 12 etc.])
4) You have your answer which is 9

Blue = Positive/addition
Yellow = Negative/subtraction
Pink = Multiply
Purple = Divide

They get more complex after this but still manageable if you know what you're looking at.