We know that A'B'C' is a rotation of the triangle ABC 90 decrees about the origin.
We remember that in a rotation, the lengths of the segments don't change, and thus, the measure of the segments are:
[tex]\begin{gathered} AB=A^{\prime}B^{\prime} \\ AC=A^{\prime}C^{\prime} \\ BC=B^{\prime}C^{\prime} \end{gathered}[/tex]
This means that the length of A'C' is 5 units, as this is the length of the segment AC.
