Final answer:
The result of multiplying (9 x 10^9) by (-2 x 10^-3) is -18 x 10^6, which is already in proper scientific notation and matches option a) -18 x 10^6.
Step-by-step explanation:
This is asking for the product of two numbers expressed in scientific notation: (9 x 109) and (-2 x 10-3). To find the product, you multiply the coefficients (9 and -2) and then add the exponents (9 and -3). Multiplying the coefficients gives you 9 * -2 = -18. Adding the exponents gives you 9 + (-3) = 6. Combining these, you get -18 x 106, which is already in proper scientific notation. Therefore, the answer to the student's question is (9 x 109) x (-2 x 10-3) = -18 x 106, which corresponds to option a) -18 x 106.