Your midpoint is quite wrong. In fact, the point you have found (how, I don't know) does not even lie on the line. If you plotted the two given points and joined them with a straight line you'd realise this immediately.

But why bother finding and using the midpoint??? You can use any point on the line, including (0, 7) which is the y-intercept. And you know y = mx + c .......