



 
Rose, There was a typo in your first problem, I hope my edit produced the correct problem. For the first problem I would use the substitution method. You already know from the first problem that y = 7  x. If you substitute 7  x for y in the second equation then the second equation become an equation in x alone which you can solve. There is an example in my response to Amber a few days ago. You can approach your second problem the same way. If you add y to each side of the second equation it becomes x = 3 + y. You can then substitute 3 + y for x in the first equation so that the first equation becomes an equation in y alone which you can solve. There is however a second method called the elimination method that you can use on any of your problems so I am going to illustrate it to you using your second problem,
First multiply both sides of the second equation by 5. Then equations then become
Why did I multiply by 5? Look at the y terms. The coefficient of y in the first equation is 5 and the coefficient in the second equation is 5. Hence if I add the two equations the y terms will be eliminated. This is why I multiplied by 5. Adding the two equations I get
Solve this equation for x and substitute the value you obtain for x into the equation x  y = 3 to find the value of y. Finally look at your third problem. The coefficient of x in each of the equations is 1 so if you subtract the two equation you will eliminate the x terms. Subtracting the two equations I get
What happened? Zero is not 1! This means that there is no solution to your third problem. Think of it graphically. Each of the two equations is a straight line and if you put them in the form y = mx + b you can see that they each have the same slope, 2, but different yintercepts. Hence they are two different parallel lines and thus don't intersect. I hope this helps,  


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