3x+y=11 and - 2x+y=1 in subsitution

-2 (3x+y=1) 3 (-2x+y=1)

-6x-2y=-2. - 6x+3y=3

-6x-2y=-2

-6x+3y=3 (change signs - 6 becomes +, + 3 becomes - 3 and 3 becomes - 3) - 6 and - 6 cancels so,

-5y = - 5

y=1

u can pick any of them but I'm using 3x+y=1. everytime u see y you fill in 1

3x+1=1

3x=1-1 (it becomes - 1 because u moved it from the other side)

3x=0

x = 0 divided by 3

x=0