this is a 7th grade advanced math class.

i have to finish a graded assignment tomorrow in class. my teacher told us to go home and practice with anything we need help on so we can fix it tomorrow. this is a 7th grade advanced math class. i understand the steps but iget confused at the end of the problem. the problem on the assignment is   -6n+15=20. the purpose of the problem is to find out what the variables are.