Answer;
sunlight
The main source of energy that is shown in the equation is sunlight.
Step-by-step explanation;
Photosynthesis is the process by which plant make their own food utilizing energy from sunlight.
Solar energy from the sun is converted to chemical energy.
The sunlight, carbon dioxide and water are used to form simple sugars and oxygen.
Simple sugars contains chemical energy which may be stored in the form of starch in plants or used in the cellular energy to generate ATP which is the usable form of chemical energy by the cells.