To solve this we will use the midpoint formula. The midpoint formula states that:
The midpoint of two points,
and
is the point O found by the following formula:
x coordinate of O: 
y coordinate of O: 
Now, let us apply this formula in our question to find the location of the enpoint G. In our case the mid point is already given as O(3,-1.5). Thus, the x coordinate of O is 3 and the y coordinate of O is -1.5.
Also,
and

Applying the formula we will get two sets of equations. The first one will be:

Therefore, 

Likewise, we can find the
as:



Thus, the location of the endpoint G is (8,-6)
Please find the attached file to get a complete picture of the solution of this question.