1. **Stating the problem:** We need to find the length of the diagonal of a rectangle with length 8 and width 6. 2. **Formula used:** The diagonal $d$ of a rectangle can be found using the Pythagorean theorem: $$d = \sqrt{l^2 + w^2}$$ where $l$ is the length and $w$ is the width. 3. **Applying the formula:** Substitute $l=8$ and $w=6$: $$d = \sqrt{8^2 + 6^2}$$ 4. **Calculating squares:** $$8^2 = 64$$ $$6^2 = 36$$ 5. **Sum inside the square root:** $$64 + 36 = 100$$ 6. **Square root:** $$d = \sqrt{100} = 10$$ 7. **Answer:** The length of the diagonal is $10$ units.