Since A'B' is the dilation of line AB by a factor of 3 with center of dilation as origin O, [tex]\frac{OB'}{OB}[/tex] = [tex]\frac{OA'}{OA} = 3[/tex] , Where A' stays on extended line of OA and B' stays on extended line of OB.
But always A'B' and AB will be parallel which means
Slope of A'B' = slope of AB = [tex]\frac{3-0}{5-1} = \frac{-3}{4}[/tex]
Hence slope of A'B' = -3/4