The composite figure has a missing length of 3.539 units.
How to determine the missing length of a composite figure
In this problem we need to determine the missing length of a composite figure, this can be done by means of known dimensions and trigonometric functions. The length formula is introduced below:
x = 8.500 - 2 · 0.750 - x₁ - x₂
Where:

x₁ = 3.113

x₂ = 0.348
Then, the missing length of the composite figure is:
x = 8.500 - 2 · 0.750 - 3.113 - 0.348
x = 3.539
The composite figure has a length equal to 3.539 units.