I havent really done this in a while but I started by making the line into a right triangle and counted how many boxes were inside(6 on the bottom 4 on the side). Then I used the Pythagorean theorem (which is the process of taking the sides and making them squares) < To simplify that explanation (I squared the sides(6 and 4).) so now we are left with 6² + 4² = x². next what you do is simplify your terms. (6² = 36, 4² = 24. 36 + 24 = 60) We have 60 = x² but we aren't don't yet, now we have to remove the square from x. and to do that we have to square root 60 and x² ( squared numbers being square rooted just leaves the regular number.) when square rooting 60 you are left with 7.8≈.
I hope this helps you, like i said its been a while since ive done this.
