A classroom had x students. Then 9 of them went home. There are now 27 students in the classroom.

A classroom had x students. Then 9 of them went home. There are now 27 students in the classroom.

Take this one piece at a time:

• We start with x students
• 9 of them went home. This means we have 9 less students. So we subtract 9 from x: x - 9
• The phrase there are now means an equation, so we set x - 9 equal to 27

x - 9 = 27

To solve this equation for x, we type it in our search engine and we get:
x = 36