
To simplify any expression you have to remember the following:
Multiplications and divisions come first, sum and subtractions come second.
This means that you have to solve the divisions first, and then you add the results:
Starting from left to rigth
First step is to simplify the fraction

Step two is to solve the division: -10÷2

Finally you have to subtract 5 from 10/5

The error was made in step 4, where the sum was made before the division.