When a quadrilateral is dilated by a factor, all its measures are multiplied by that factor.
So, the side DA multiplied by the dilatation factor, 3/4, will be the measure of the side D'A', which is 30.
Thus:
[tex]\begin{gathered} DA\cdot f=D^{\prime}A^{\prime} \\ DA\cdot\frac{3}{4}=30 \\ DA=\frac{4}{3}\cdot30=4\cdot10=40 \end{gathered}[/tex]
So, the measure of side DA is 40.
