Subtraction is taking something away. (Addition is adding something.)
Count these cars.
How many are there?
Did you get 10?
The 7 cars below drive away.
How many cars are left?
10 - 7 = 3
You could use your fingers to help you with this this - start with 10 fingers up, and put down 7.
How do you feel about tackling some questions on this?