so you subsitute y=2/3x-4 for y in 2x+y=4 so
2x+(2/3x-4)=4
2x+2/3x-4=4
add like terms
2 and 2/3x-4=4
add 4 to both sidees
2 and 2/3x=8
2 and 2/3=6/3+2/3=8/3
8/3x=8
multiply both sides by 3/8 (makes 8/3=24/24=1/1=1)
x=(3*8)/8
simplify
x=3
subtitute x=3 into y=2/3x-4
y=2/3(3)-4
y=(6/3)-4
y=2-4
y=-2