Question 244256
y=2x+7
3x-y=-9
add y to both sides
add 9 to both sides
3x+9=y
equate the two
3x+9=2x+7
substract 2x from both sides
x+9=7
substract 9 from both sides
x=-2
y=2(-2)+7
y=3
check

3(-2)-3=-9
-6-3=-9
-9=-9