Answer:
3.875
Explanation:
A lot of Pythagorean theorem stuff!
First, find the base of the biggest triangle:

Next up, find the base of the second biggest triangle:

At the moment we still don't know x, so we will later need to come back to make sure that this expression is positive.
Next up, the smallest triangle:

And we are done!... or not...?
We still need to go back to the second equation to make sure everything is positive:

Whew, thankfully everything is positive.
Now for the second time, we are now done.