and .
Substituting the second into the first gives
Now solve for and substitute back to find the corresponding values. This will give you the two points, so you can use the midpoint rule to find the midpoint.
The kind of obvious thing to do is to find the two point of intersection. From 2y+ 3x= 9, y= 9/2- (3/2)x. Replace y in and you get a quadratic equation for x which should have two solutions. Once you have found those two points of intersection, find the point midway between them.
Do you know that the point midway between and is ?