To calculate the coordinates of a point in a figure when dilation is applied, you just have to multiply the x and y coordinates by the scale factor (n), like this:
P(x,y)->P'(n*x,n*y)
In this case, the scale factor is 3, and the original point A(3,6), after the dilation the point A becomes A'(3*3,6*3)=A'(9,18)
Then the point A' of the scaled rectangle has the coordinates (9,18)
