I assume that T (0,-4) means a translation by (0,-4) and D4 means a dilation by a factor 4. Dilations are performed from a specific point, and since none is given, we'll assume the dilation if from the origin (0,0). In general, transformations are not commutative, meaning the order you perform them is important. Doing the dilation first, then the translation will not yield the same answer as doing the translation first followed by the dilation. Since you specified T(0,-4)*D4, we do the translation first:
T(0,-4)
(-8,6) ------------> (-8+0, 6-4) = (-8,2)
Now do the dilation:
D4 (origin)
(-8,2) ----------------> (-8*4,2*4) = (-32,8)
