(19)4.5 x (19)? = 199

Key idea: when multiplying powers with the same base, add the exponents.

Start with the equation written with exponents:

• 19^(4.5) × 19^(?) = 19^9

• Use the rule a^m × a^n = a^(m+n): therefore 4.5 + ? = 9.

• Solve for ?: ? = 9 − 4.5 = 4.5.

Conclusion: The missing exponent is 4.5, which matches the correct choice.