If the point (3, k) is on the graph of the equation x + 2y=15, what is the value of k?

Because you know the x value of the coordinate point on the graph is 3, to fins k, you must plug the value of 3 into x and solve for y to find k. The reason for this is because k is the y value of the coordinate. When you plug the x value into the equation, it should look like this:
3+2y=15

Then, you subtract both sides by 3 to isolate 2y and you get 2y=12. Finally, when you divide both sides by 2, you get y=6 which means your answer is 
k=6