Find the midpoint of the two intersection points of the graphs 2x-3y=7 and (x-6y)^2+9(3y-2)=-5.

I could not understand the following method...

Let the two intersection points be (X1, Y1) and (X2, Y2).

Then X1+X2=22/3 and they work out Y1+Y2 using the value of X1+X2 and finally divide by 2 to get the midpoint.

My question is how to get X1+X2=22/3?

Your help will be much appreciated!