Answer: a = -250
Explanation:
1. Solve the left side: 0.5a + 0.5a = a
2. Subtract 0.9a to the other side which gives: a - 0.9a = -25
3. Solve the left side again: 0.1a = -25
4. Divide 0.1a to the right side to isolate a: a = -25/0.1
5. Now divide -25 / 0.1 to get a = -250