I can't tell what the scale factors are from your problem statement, but let's call it k. For a given point (x,y), the rule for dilating about the origin is:
dilate by k about (0,0)
(x,y) ------------------------------> (kx,ky)
where (kx,ky) is the image of (x,y) after the dilation. For example, if k = 3 and (x,y) = (1/2, -2), the dilated point would be ( 3·(1/2), 3·(-2) ) = (3/2,-6).