x = -8
The sum of half a number is:
x/2 + 13
Where x can be any number, we divide x by 2 to get half of it.
This is equal to 9, so we can say
x/2 + 13 = 9
Now we can solve this equation to find out our starting number, as we have one variable.
x/2 + 13 = 9
Multiply by 2
x + 26 = 18
Subtract 26
x = -8
