In order to check if the x and y value is a solution then you need to plug in your numbers for the x and why value.
Take this as an example with your solution (4,7) let's try 4x-9=y. We want to plug in the values of each variable so the new equation will look like 4(4)-9=(7). Now you want to multiply and get 16-9=7 next subtract and you get 7=7. This would be a solution for the problem.
Now we can try 3x+14=y do the same thing from last example and you'll get 3(4)-14=7 and now multiply to get 12-14=7 now subtract and you'll get -2=7 which is NOT a solution to this equation so it would become -2≠7.
