1. **State the problem:** We have a triangle with three interior angles: 71°, 73°, and 18a°. We need to find the value of $a$. 2. **Recall the triangle angle sum rule:** The sum of the interior angles of any triangle is always 180°. This means: $$71 + 73 + 18a = 180$$ 3. **Set up the equation:** $$71 + 73 + 18a = 180$$ 4. **Simplify the known angles:** $$144 + 18a = 180$$ 5. **Isolate $18a$:** $$18a = 180 - 144$$ $$18a = 36$$ 6. **Solve for $a$:** $$a = \frac{36}{18} = 2$$ **Final answer:** $$a = 2$$