Answer:
use distance formula
[tex]\qquad\quad {:}\longmapsto\sf AB=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}[/tex]
[tex]\qquad\quad {:}\longmapsto\sf \sqrt {(4-8)^2+(9-1)^2}[/tex]
[tex]\qquad\quad {:}\longmapsto\sf \sqrt{(-4)^2+(8)^2}[/tex]
[tex]\qquad\quad {:}\longmapsto\sf \sqrt {16+64}[/tex]
[tex]\qquad\quad {:}\longmapsto\sf \sqrt {80}[/tex]
[tex]\qquad\quad {:}\longmapsto\sf AB=4\sqrt{5} \:or \:8.94[/tex]
