G divides each median in the ratio of 2 to 1 (the longer side goes from G to the angle)
triangle AMN is similar to triangle ABC (why?)
AMN is a scaled version of ABC (by a factor of ⅔)
its area should be scaled by (⅔)^2
you take the points (0,6) and (4,-10), then you subtract -10 by 6 and you get -16. then you take 4 subtracted by 0 and you get 4. -16 divided by 4 is -4.