Hi,
I am bad with the mathematics, but I would say the solution would be a number cus' when you try to calculate it you have :
x-3 = 9
⇔ x-3-9 = 0
⇔ x-12 = 0
⇔ x = 12
The result is 12, so it really means that x is just a number and not a variable. A variable would be another letter as result, representing a number that could change if we changed some other numbers in the equation.
What do you think ?
