Explanation:
There are several ways to solve this, but the only one I remember how to do is with the pythagorean theorem. The distance between A and B is the same as a hypotenuse of a triangle with a height the absolute value of y1-y2 and a base of the absolute value of x1-x2.
For example, in the first question, the coordinates are (1,2) and (9,8), so we have a triangle with base 8(9 - 1) and 6(8 - 2). Now we just need to plug that into the pythagorean theorem like so:

or plugging our numbers in,

The answer is the radical of the answer, in this case 10.
So if this makes sense, go ahead and try to work the next three out on your own, and if you still need help let me know <:
Hope this helps Bucke!