Respuesta :

First, find length of AB, using coordinates of the points A and B.
A(1,4) and B(4,8)
|AB|=√((y2-y1)² +(x2-x1)²) =√((8-4)²+(4-1)²)=√(16+9)=√25=5

When dilating with scale factor 4,
the length of |A'B'|=4*|AB|=4*5=20