Okay I'll try to show it step by step
Step 1. So first you need to recognise that the diagram is a right angled triangle, with the right angle being at the bottom. This means we can do Pythagoras' Theorem.
Step 2. Now that you know you can use Pythagoras' Theorem, you can apply it. If you don't know, the theorem is a² + b² = c², with, in this case, a being 5 and b being 8. This means we can do 5² + 8² = 25 + 64 = 89.
Step 3. From the previous step, we now know that the length of the side including x is √89, and because we know that part of this side is 3, if we subtract 3 from √89 we get the value of x, so: √89 - 3 = 9.434 - 3 = 6.434, which is your answer.
I hope this helps!
