What must be true is the expression will actually be adding 10 + (+ x), because of the two marks you have to make when subtracting by a negative.
The subtraction sign is also the negative sign. So in this case, the expression is also saying -x, which means there is two negative signs.
A double negative is a positive, so in this case the x will be turned to a positive integer.
If this is still too confusing, she can plug in a number to replace x. Let’s try it with -2:
Ex. 10 - (-2)= 12.
The two negative signs turn into one positive sign, AKA an addition sign. So, the above expression can be simplified as 10 + 2, or 12.