Please help me with this and if you can explain please, okay so i have x-y=0 and x-2y=6 i have to solve by using substitution, at first i though i can replace x-2y=6 with x but then i notice that i can't can i put one of the equations into y=? Please help me with this thank you

from x-y=0 then x=y.substitute into

x-2y=6 it will be y-2y=6,which y= -6 and since x=y then x= -6