Answer:
Hypothesis: I believe that plant food x will increase the production of tomatoes more than tomatoes without plant food x.
Controlled experiment: Get 2 tomatoes plants that are the same age and height. One tomato plant will have plant food x and the other will not have plant food x. For 2 weeks, give both plants the same amount of water, sunlight, and air temperature (a small greenhouse can keep the experiment controlled). After the weeks are over, measure both plants to see which one grew taller and then gather all of the information to see if your hypothesis was correct.