Just replace 13 for x in each of the possible equations.
1. 13+3=10
Since this is false, this is not the answer.
2. 13-10=3
Since this is true, this is the answer.
3. 13/2=26
Since this is false, this is not the answer.
4. 13+12=1
Since this is false, this is not the answer.
The answer is 2. x-10=3, since x=13 makes it 13-10=3, which is true.
