Since you already plugged in the x solve for the y. For example on the third problem: 2(0)+4y=8. First solve 2 times 0 which is 0. Now you will only have 4y=8. From here you would have to get rid of anything next to the X and that number is 4. As you know 8 and 4 can both be divided by 4. This will leave you with y=4 and that is your answer. Tell me if you need to explain it in any different way for you to better understand!
