Normally, when I do this, I differentiate each term first with respect to x then with respect to y. In this solution, I differentiated the entire expression with respect to x, then with respect to y. That makes separating the dx and dy coefficients much easier, so the solution almost falls into your lap.
A. They are both proportional because they have the same ratio. b. The two graphs are the same because they have a constant rate of change (50), but they have different y-intercepts.