What is the length of the longest stick?
The longest stick is equal to the length of the 4-stick, plus the 1-stick, plus however much the 6-stick extends beyond the 1-stick.
The left end of the 6-stick starts at the same point at the left end of the 3-stick. And the right end of the 3-stick ends at the same point as the right end of the 1-stick. This means the 6-stick extends 3 beyond the right end of the 1-stick. So the total length is 4 + 1 + 3 = 8.