Please go easy on me as it's 35yrs since I did this at school! I'm helping my daughter and the equation is

x^2 - y^2 =12 where y = x-2 .

By substitution I've worked the answer out as x=4 but none of the methodology (e.g. factorisation or quadratic formula) in her notes or the text books give me the right answer. Through factorisation, x = -2 which is wrong, any one able to help as this driving me nuts!!

Rgds, Mark