V minus 10 equals -9
V - 10 = -9
Add 10 to both sides of the equation to eliminate the -10 on the left hand side
V - 10 + 10 = -9 + 10
V = 10 - 9
V = 1
Re-writing the expresion as 10 - 9 does not change the answer at all. Instead its just a matter of re-arranging the expressions to avoid any likelihood of getting the signs confused. In short,
-9 + 10 is the same as 10 - 9