It looks like you have to do distance formula. It's basically this:
D =
(ignore the A, i dont know how to get rid of it sorry)
I would then plug in the points of A(1,7) and B(10,3).
D =
Then I would get the square root of 4^2 + 9^2
= square root of 16 + 81
= square root of 97
So the length of AB would be the square root of 97