When you write an equation, it tells you, that you know what the math problem is and your steps of solving. For example if the math problem was that:
Trina invited 9 girls and 10 boys to her birthday. 2 girls didn't come and 1 boy didn't come. What is the total amount of people that came to Trina's birthday party.
Equation: 9+10-2-1=?
Answer would be: 9+10=19
19-4=15.
So only 15 people came to Trina's birthday party!
:) Hope I helped!