To solve for x, we need to simplify the left-hand side of the equation:
3x + 9 - 8x = -31
Combining like terms, we get:
-5x + 9 = -31
Subtracting 9 from both sides, we get:
-5x = -40
Dividing both sides by -5, we get:
x = 8
Therefore, the solution is x = 8. We cannot simplify this any further.