In order to identiy which is the dependet quantity and independent quantity, you consider that an independent quanity is that quantity does not depend of any other. Insteas of that, dependent quantity depends of the values of other quantity.
In the given case, you have that an increase in cardio exercise results in a decrease in weight. In this case you can notice that the weight depends of the cardio exercide. Hence, cardio exercise is the independent quantity, and weitgh is the dependent quantity.
