change x+2y=-4 so x is alone
x+2y=-4
x+2y-2y=-4-2y
x=-4-2y
substitute -4-2y into x in y=-1/2x-4
y=-1/2(-4-2y)-4
multiply 1/2 with everything inside the parentheses
y=-1/2(-4)-1/2(-2y)-4
y=2+1y-4
simplify
y=1y-2
remove 1y on both sides
y-y=1y-y-2
0=-2
no solution
