Points A(-8,3) and B(1,-9) are the initial and that terminal points of line AB. Find the magnitude of line AB.

You know pythagoras' theorem don't you?

Draw the points on a graph, and draw a right angled triangle with the line AB as the hypotenuse, and the other 2 sides in the x and y directions respectively.

You can work out what both of the other sides are can't you? So you can also work out how long the hypotenuse is!