The rule for dilation (scale factor k) about the origin is given by:
(x,y)->(kx,ky)
The scale factor k can be obtained from any one of the ratios:
One pair of adjacent sides are congruent (WZ and ZY) so as WXYZ is a parallelogram all sides are congruent.
We immediately note that 3 and 9 are multiples. Therefore, they are factorable. 9 goes into 3 three times, and 3 goes into three 1 time. Therefore, we can take out three from the whole expression to get 3(c - 3).