The two points mentioned in the problem are represented in the following image:
We are told that the triangle must be an isosceles right triangle, this means that:
to be an isosceles it must have 2 equal legs
to be a right triangle it must have a 90° angle.
In the previous image we can see that the vertical leg has a length of 8.
So the other leg must also have a length of 8:
As we can see we have the two things: two equal sides (isosceles ) and a 90° angle ( right triangle).
Thus, the longest side must be the hypotenuse of the triangle, which we find using the Pythagora's theorem:
To find the hypotenuse with Pythagora's theorem we take the square root of the legs squared as we see in the previous image.
Now we solve that to find the length:
Answer: the length of the longest side of the tip is